Not needed if you come up slowly but I always do it. If for no other reason then to hang out and stay down as long as possible. It is a good place to just relax after the dive or practice.
Thalassanamia (sp) has a good explanation as to why the safety stops were originally put into the OW course and I think the summarized-uneducated version is 'to slow you down'
I think this is the post OldNSalty was referring to:
Let's be clear: A slow ascent will not hurt unless it is so slow that you start picking up gas.
The reality is that "Safety Stops" are antique artifacts that the recreational diving community seem to be unable to shed. In this day of 30 FPM, computer controlled ascents "safety stops" should become irrelevant. I say that as one of those who was in on originating them. The idea of safety stops came out of some research that showed that recreational divers were, in point of fact, making their normal ascents between 100 and 120 feet per minute, back when a standard ascent was 60 fpm.
The ascent problem was noted by the National Underwater Accident Data Center, it was investigated and confirmed by Glen Egstrom at UCLA, the safety stop was first suggested by Andy Pilmanis of the USC Catalina Lab Chamber back in the 1970s. It was first adopted by NAUI after the AAUS Decompression Computer Workshop where Bruce Bassett (as I recall) suggested perhaps a stop between 20 and 10 feet for two to three minutes would be every bit as effective as actually getting divers to slow down to 60 fpm. In addition it would also cover most errors of one depth too deep or one time too long.
So the "safety stop" was, in reality, a "cover" for divers' refusal (or inability)to slow their ascents in the pre-computer days. Anyway ... back then 60 fpm was the standard ascent rate and we all used Navy tables, If you knew what you were doing you came up 5 feet on your depth gauge/5 seconds on your watch.
At the same time that computers started to appear there was a flowering of tables, Harry Averill's brilliant reformatting of the Navy Tables into a circular design for NAUI, Karl Huggins' development of the Spenser No-Bubble model into a set of tables, PADI's first RDP, etc. Then two things happened, first was that all the agencies but PADI took the NAVY tables, cut them back a stop or two (for no scientific reason, just a "less is better" legal mentality) and put them out with their own logo on them (PADI came out with their RDP, a good piece of work by Ray Rodgers); second was the institution of the safety stop, which also had not tested scientific basis, just, once again, a "less is better" mind set and procedure patch, but a sensible one given the ascent rate problem.
The model Ray used for the RDP predicted the decompression required to keep the risk of DCS to a negligible level and included divers trying their best to ascend at a constant rate of 60ft./minute. PADI tested this with Dopplers and found that their approach was (I believe) a "no-bubble" solution. The reality is that with a 30 FPM ascent you're not going to have detectable bubbles anyway. If you are using 60 FPM based tables (and remember that the original U.S. Navy tables were almost based on 120 FPM to satisfy the needs of Doug Fane's UDT folks) Spenser did show that a 60 fpm ascent following 50 min at 2.8 ata would exhibit bubbles in some cases. But ... I believe that the PADI tables were doppler tested and a 60 fpm ascent within the limits of those tables did not show bubbles, thus demonstrating that making such a stop a waste of time and a singularly foolish criterion for emergency ascent planning. The idea of the divers trying their best to stop at 15', then trying their best to rise to the surface under control, well ... that was added by the lawyers looking at what other divers where doing (as a solution to the ascent rate problem) not by anyone with a background in decompression.
Well ... computers unexpectedly solved the ascent rate problem. Computers permitted very slow ascents and accounted for the possibility of gas uptake on such ascents and also provided an ascent speed indicator that slowed divers down, since most computers by that time required a 45 or even 30 fpm ascent rate. This was, in my mind, a wonderful thing. But that left us with divers who were performing a now unnecessary step, the "safety stop" and an instructor community with too little real knowledge of decompression to see that the "safety stop's" time was over (not to mention the fact that PADI was locked into a 60 fpm ascent rate and reasonably wanted to maintain the safety stops at least at the heavier exposure end of their RDP). So now PADI continues to push "safety stops" without any clear explanation of why and the "safety stop" seems to be rather indelibly inscribed in the sports diving community's procedures. Once again, from a decompression perspective a safety stop will not hurt you, and can only help (however minimally), but from an operational and procedural prospective can be problematic. The reason for the stop has been forgotten, just the procedure itself is remembered.
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Permit me to add that deep stops (1 minute at half your maximum depth) have been shown to be a good idea and that faster ascents, when coupled with longer shallow stops, may be a better approach than a slow gradual ascent.
