you could wire direct to battery but if you forget to switch them off, or a fiddling oik switches them back on for you, then you will soon get a flat battery. Better to either splice into an existing switched circuit that can take the additional load (radiator fan live is a good one as its unlikely the fan will be on at the same time as your heated grips); or even better, both splice into a switched circuit AND feed the grips via a relay. If you use a relay you can splice into any switched circuit regardless of the current it would normally draw as you are only using that circuit to trigger the relay.

Good luck with the install. I use Dr Bike grips on the basis that they are the cheapest I can find and seem to be a copy of old model Oxford heated grips that are twice the price. ALso a nice easy to use dial type switch. J&S accessories.