As a fire mage

[:1]Should I let the 2p build up to 10 before my first combustion for the 4p bonus or just use combustion as soon as I get a decent ignite?
Use it as soon as you can and let the new cooldown start.
Depends™

Whether you blow it immediately or wait for a 10stack, your 2nd ignite is gonna come at about the same time. If there's a good reason to delay it a bit, like zonozz's increased damage phase, or an add phase that would let you combust->impact, definitely delay it. If there's a good reason to use it ASAP vs later, use it asap.