For most of my 27/72/75 MHz transmitters and my Spektrum DX7 that take 8AAs or 8AA packs, I use rechargeables.

For my 2.4GHz DSM2 radios that only take 4AAs, I just feed them alkalines (a DX6i and two mini TXs from RTF Parkzone Vapor & Ember). I can get 4AA alkaline batts for less than $1, and they have a long shelf life. The DSM2 radios I use them in will run quite awhile on them too. Rechargeables will work fine in those as well, but they don't go through the alkaline AAs fast enough for me to worry about the cost.