For myself, the only reason to copy a 7.5" tape is to preserve/archive it. When archiving, you want to do the least damage possible to the sound, so 15/IEC would be the choice. If you ever want just to listen to the tape, playing the original will always sound best (of course). One of the few weaknesses of RTR is copy degradation. It's a lot unless you use a ridiculously good format like one inch half-track! (TTP's method)
BTW, it's a lot more difficult for a RTR to have good mechanical performance (wow, flutter,etc.) at 7.5ips. Decks needs to be well-designed and in like-new condition.
Tim