Should I use a dedicated circuit for a towel warmer?
This is a pretty small load, so it probably won't be an impediment, but you need to follow all the rules for wiring in bathrooms. 

If the circuit powers hardwired loads only, this with the other loads can't exceed 80% of total capacity.   

If the circuit powers hardwired loads and receptacles, the hardwired loads can't account for over 50% of capacity. 

If the circuit powers receptacles in the bathroom, then it can only power things in that same bathroom.  NEC 210.11C3