My lighting is AC. The voltage regulator makes DC to charge the battery to run the starter. The DC circuit has the odometer computer, the horn and the cooling fan as well. I can't run my lights with the motor off, but I also can't forget to turn something off and kill the battery. I do have a switch on the headlight so I can turn it off so the charging system can be dedicated to topping the battery or running the cooling fan.
I didn't bother with a headlight relay. I recently went from a 34watt H4 bulb to a 55watt H4 bulb. It should draw about 4.5 amps if the AC is around 12V. I think the switches are up to it. I actually have two switches: an on/off and a high/low. The 55watt is noticeably brighter than the 35w was. My reflector and lens is def' not a high performance setup but works for now.
Whatever you end up doing, make sure the connections are solid. They will get wet, dusty and be subjected to a lot of vibration. Low cost relays may cut the reliability of the system. Keep it simple so it is sure to function when you need it.
'03 KTM 450EXC
"If you know what you're doing, then you aren't learning anything." - A. Lincoln