If you're carrying less than 30lbs I'd say you're good with rear bags only. More than 40lbs, definitely go the 4-bag route. Between 30 and 40, take your pick; but the heavier you go, the more the handling will benefit from putting a reasonable amount of the weight up front.

40lbs is about as heavy as I've gone. 25-30 rear, 10-15 front worked well for me and provided a very stable and predictable ride.

EDIT. You don't need a huge weight in tools. A multi-tool, a chainbreaker, a few spare links, a few spare spokes, spare brake and gear cables, a spoke wrench, some tyre levers, a couple of spare tubes and a patch kit is about all. That's been sufficient for me on an 8-week trip, anyway.