Thats a very good question. Answer - it depends.
The reason you've learned to use increased percentages of Oxygen, i.e. EAN mixes, is to offset the negative affects of exposure to Nitrogen. It's the Nitrogen which limits our dive times due to it's absorption into and then time to be released from our bodies. So higher EAN and higher Partial Pressures gives us more time (though not more depth!).
But like so many things in life, nothing is free... by increasing Oxygen in your breathing mix, i.e. increasing the partial pressure of the O2 in your mix, we are exposed to a new risk of Oxygen Toxicity. There are 2 kinds, the main one we're worried about is CNS or Central Nervous System toxicity. As you probably recall... once the partial pressure of O2 in your mixes reaches a certain level, by diving deeper and the partial pressure builds, you increase dramatically the chance of rapidly and with little warning suffering from sudden seizures from the O2 toxicity. While generally not life threatening above the water, underwater your very likely to drown. The safe threshold for not having this happen is something above 1.6... the military sometimes pushes the limits above this, some training course use a 1.6 threshold, others a more conservative 1.5, or 1.4 (PADI).
So simple answer is 1.6 or below, with 1.4 being more conservative. Never go above 1.6.
A fuller answer lies in another factor in using higher percentages of O2, and that is cumulative Oxygen Pressure limits... which are simply a reality that your body really isn't designed to handle high concentrations of O2 for long periods of time - so there are limits. At a partial pressure of 1.6 O2 you're limited on a single dive to 45 minutes of exposure, wheres at 1.4 your limited to 150 minutes (NOAA table). For cumulative dives each minute at a partial pressure of O2 at 1.6 'uses up' if you will 2.2 percent of your daily limit, whereas at 1.4 you only 'use up' .67 per minute of your daily limit. Once you hit 100% of your limit your done (most recommend stopping at 80% of your daily limit). So you can dive much longer (more dives per day) using a lower partial pressure of 1.4 (or 1.5) than 1.6. It'd be hard to hit the limit using 1.4 - which is why it is the more failsafe recommendation.
What you should consider when diving enriched air... first know the Maximum Depth you can go to (which is the depth at which the partial pressure of O2 equals the limit your shooting for; 1.4, 1.5 or 1.6). Use your tables or the formula MOD = ((1.#/EAN##)-1)*33 Example: (1.4/.32)-1*33=111 Max Depth for EAN32 at 1.4 PPO.
The second thing then is account for, at least generally, how much diving your going to do on O2 at depth all day... if your pushing the depth limits (i.e. breathing high partial pressures of O2) and doing multiple dives - then be conservative and use 1.4. If you doing one dive, have a long surface interval, have a large tank with lots of air and are diving to depths more half way to your MOD, then a higher limit of 1.5 or 1.6 may be worth considering. If in doubt, use 1.4.
Short answer though... use 1.4 for now... to help make this all make sense I'd highly recommend the Advanced Nitrox class to really understand the trade offs of replacing Nitrogen with Oxygen. Its not a hard class a tall... just grounds you in how trading of Oxygen to reduce Nitrogen has new issues to consider. Once you 'get' this, the tradeoffs involved, at least from my point of view , you feel much more confident of what breathing enriched air is doing for and to you and your well on your way to understanding similar issues in decompression planning and even more advance mixing of gases (trimix - where you go one step further and offset the O2 toxicity with inert Helium... but thats another challenge for another day.)
