I struggled with this as well this summer. I primarily used my actor/asm to help set up and take down each rehearsal. She was also super helpful and jumped up to help reset if we were running a moment again. This was especially great when running a certain number with a lot of prop choreography.
However, I didn't feel right asking her to come in early and help set up on days where the calls were staggered and her character wouldn't be needed at the top of day. I also made sure to respect her break times, so she wouldn't go without a break trying to pull double duty.
Once we got into the theater I didn't really use her at all - according to the TYA agreement, all ASM duties must be complete by half hour, so anything additional (set moves, etc) would fall under the same additional compensation as a regular actor. I had an intern and a wardrobe crew member backstage, so I didn't feel like I needed the actor/ASM to help out prior to half hour.
I'm sure that an Actor/ASM is invaluable in a touring situation, but it's a little less obvious how to make good use of them in a resident theater.