I have one of the older gen Rockets. I installed a voltmeter on my ride so that I could monitor things happening before the battery goes dead. With headlights on and the Triumph heated grips on high, there is no appreciable drop in voltage. I guess it helps that I also have led bulbs in my headlights, driving lights and tail light but even when I had halogens, my voltage never raised a concern. Also, I run my heated grips at least 60% of the time. On some long trips, I never turn them off. They are usually set on the low setting (only 2 on the Triumph grips) with never a problem with battery draw...
Just did a quick Google search and it looks like most grips draw from 3 to 4 amps. Shouldn't really cause a good charging system any problems.