I think Ken is correct in that many snakes, and other reptiles, will do fine at a relatively constant temperature. That doesn't necessarily mean that a gradient isn't desirable if you can create one. We heat our room to ~24 C and then use individual heat pads/tape/rope/basking lights as appropriate to create warmer areas.
Belly heat vs. ambient heat: If you heat the room to a certain temp., than that will be the heat of the snakes belly! Rocks, water dishes, and everything else will be about the same temperature. This also means that snakes can't go into the water dish to cool off- the water will be the same temp. as the room, and hence the snake. Water at the same temp. as the air only feels colder to us because we're endotherms!
Having said this, it is more expensive and wasteful of resources to heat an entire room to 32 C than it is to heat the room to the appropriate cooler temp. and use individual heat for certain spots.