I try to keep my betta tanks at 80 degrees. I use a digital kitchen thermometer to measure the temperature. This thermometer doesn't stay in the tank all the time. I only use it on water change day, and sometimes when I feed the fish I do a quick reading. My latest readings ranged from 80.4 - 81.6. I felt that 81.6 was a bit too warm so I nudged the heater down a bit and I'll get a new reading today.
I would say that 82 degrees is the warmest temperature that the betta should be kept in, barring an ich treatment. I have read that the higher temperatures will speed up the fish's metabolism, possibly to the extent of shortening their life. On the other hand, keeping a betta in 74 degree water will probably lengthen their life span, but they won't be comfortable and happy. I think 80-82 degrees should be your target temperature.