I've never been tipped, at least that I know of, as I am usually never around when the regs is picked up and paid for.
I've always worked for 50% of the shop's labor charge - so if the shop got $25 per stage, I was paid $12.50. That's been the case with 4 separate shops over the years.
On some regs, like a Mk 2 R190 with a R190 octo, it's a great deal as those regs can be serviced very quickly - in about an hour, cleaning replacing all the annual service parts and then bench testing without cutting any corners. On a more involved first and second stage, and/or one with issues, it can be a lot less of a deal. A regulator that is difficult or time consuming to disassemble, assemble and/or adjust can take twice that long. If it also has fittings that tend to stick together, it will take even longer and if you add in an inordinate amount of corrosion or dirt requiring extra cleaning, you end up with maybe 3 times the base time in the reg. I charge extra for the exceptionally corroded, dirty and abused regs, but again I lose half of that charge back to the shop (that in this case did nothing) so I mostly eat the extra time and just accept that it's the payback for the easy to service regs and that over time it all averages out.
A potential problem with that system however is that it can place the technician in a position of making less money per hour on the most challenging regs, and if he or she does not take a big picture view of things, the temptation to cut corners becomes large.
In some areas however I suspect the tech is just paid an hourly wage. I recently moved and I suspect thats the case where I now live, as they have a large supply of local college students with diving interests who would be willing to work for a low hourly wage, just to be in the business. Given the labor charges are similar, the shop will make a killing even on a dirty/corroded reg. The good news is that the hourly tech has no real incentive to hurry and if it takes all day to clean and service a difficult reg, he or she just makes more money on that reg. The downside is that it's low paid work that does not attract an individual with a high level of training or experience.
That's really the rub. If you pay your tech(s) 50% of the labor you'll attract more experienced techs, but it's still just going to be a part time job so you have to work around that. The alternatives are 1) the low paid hourly tech who will generally be new to the business and generally eager but inexperienced and won't stay all that long, or 2) a shop owner/manager/employee who does reg servicing as a sideline. That can work ok if they are given blocks of time to just service regs, but if they have to do it in between customers, quality suffers and inevitably regs go out the door with "extra" parts mysteriously lying on the bench.
Personally, I'd prefer to see reg servicing move away from the local dive shop, as too many shops do an inadequate job of servicing and in general the charges tend to be high as the shop has to use it as a revenue source to stay in business due to the high overhead in a LDS. If a tech were independent and did not have to maintain the general inventory and overhead of a full service dive shop, the charge to the customers could be 1/2 of the current rate (the 50% they get now), and an on-line/mail-in business format would allow enough business to have the tech support him self or herself full time. The end result would be better quality service at about 2/3rds the current service costs. But that's not going to happen under the current model, so techs tend to be tied to the shops as the shops have access to the parts.