As long as it is consistent and working for you I wouldn't feel the need to change it.
CO2 is a liquid in your tank and the pressure will change with temperature. The vapor pressure for CO2 at 50*F is right around 650 PSI, so unless your CO2 tank is colder than that, you can just use that as the "it's getting there" point.
Your regulator won't just go "WHOOSH" as it drains out, but it will slowly increase the outlet pressure as the tank pressure decreases. This is normally as a % of the set value, so having a higher set pressure won't help you. If your regulator only goes up a couple PSI as the tank pressure drops to zero (this is possible on both single and double stage regulators depending on design) then you have nothing to worry about, unless you're right on the edge of what your fish can take. You can also counter this by adjusting your regulator every day, but that gets really old really fast.
I have a Milwaukee regulator, likely one of the cheapest designs out there. My solution was to plumb a $10 low pressure regulator after it, effectively making a two stage, and that was so that I can run the tank empty without having to make daily adjustments.
Here's a video of the pressure rise on my CO2 tank. I simulated it by shutting the valve, but the same thing happens (over a longer time scale) when I let the tank run empty:
[video=youtube;M08hPs-J3SM]http://www.youtube.com/watch?v=M08hPs-J3SM[/video]
