I have an aqueon 10 gallon heater that came in the set and a boyu sticker thermometer while the aqueon heater says it is set at 76 the thermometer says it is in the 78-80 range (Fahrenheit) which one do I trust??
Firstly (and I'm sure other members will back me up), stick-on thermometers are a guage at best, and should not be considered accurate. For a few dollars you can get a glass thermometer that is more accurate.
As for which one to trust, well that's hard. In your situation I wouldn't trust the sticker, but in general, a thermometer will be more accurate.
Heaters that are "set" (non-adjustable) are generally designed to raise the temp a few degrees above ambient, whereas adjustable ones, well, they can be adjusted. e.g. if you adjust it to 76 and it actually performs at 74, then just dial it up a bit higher.
Some heaters can be self-calibrated (EheI'm Jager), so you just 'reset' the calibration if the number is wrong.
If your water is higher than the temp on the heater, that doesn't always mean just dial it down, other factors such as lighting, size of tank, ambient temperatures could be causing the higher temp, not the heater. If your heater has an 'on light', check to see if it's coming on.
On subject, A thermometer in the filter is a good idea to me because it lets me see the temp of the water coming out of it. I have two thermometers on each tank and one is at bottom of tank and the other is either in the filter or near its output.