[T.G]

I don't have a SFB anymore, but generally speaking, I've never noticed anything detrimental from the small amount of smoke generated by having briquettes light off during a cook.

My determination of adding lit or unlit is by pit temp. If temps are low, usually because I let the coals burn too far down or for whatever reason not all lit off properly, I'll add lit coals. If it's running fine, I'll just add unlit right on top.

I try to minimize disturbances to the coal bed through actions like raking and such as it tends to kick up a lot of ash that can possibly end up on the food.