I usually attack them with whatever I have on hand in the area.

If it's a little stack of rebels, the nearest garrison of militia will usually suffice.

If it's a big enough stack I will order up some better troops first. If I happen to have a general around I'll let him fight. But then I'll usually send those troops up to the front. The new ability to build multiple units per turn eliminates the need to keep an army back to deal with rebels. You can raise as big an army as you need in a couple turns.

No need to rush, but it's too expensive to leave them around for the long term.