The subfloor joints all need supporting. In the typical installation, the long dimension of each sheet is T+G and oriented perpendicular to the joists. The short dimension joints are over a joist.
So when you cut out the subfloor flush with the bottom plate, the simple case is for a wall perpendicular to the joists. You need to splice your new subfloor to your old subfloor. You can do that in each joist bay by using a, say, 4" x 14" piece of plywood (if the joist bay is 14-1/2" wide, for modern 2x at 16" o.c.), and screwing half its width to the underside of the old subfloor under the bottom plate, with maybe 3 screws (not drywall screws). That gives you something to secure the edge of the new subfloor to.
On the walls parallel to the joists, best case is when there's a joist directly under the wall. If the joist face towards the room is flush with the edge of the bottom plate, then the old subfloor is well supported. For the new subfloor attach a 2x4 cleat to the side of the joist with the top in plane with the joist.
Worst case would be when the joist is just inside the room. In that case if you cut the subfloor back to the wall plate, you'd have no access to resupport the cut edge. So unless you have access from below, cut the subfloor back to the middle of the joist and refasten it to that joist. The joist will support your new subfloor, and you'll have a narrow strip of height difference. Add a thin layer of plywood to make it flush. That area won't be as strong, which could matter for a tile installation, particularly if it's a high traffic area.
For the intermediate case where the wall plate lies over the middle of the joist bay, I think the thing to do is to install blocking across the joist bay, at the ends of the room and say every 2' o.c. That could be tricky, but as long as enough of the joist bay is open within the room, you should be able to reach in and toe screw upright 2x4 blocks to the joists. If the wall is load bearing, you should find that full height cross blocks are already installed for you. Once it has the blocking, treat it the same as a wall perpendicular to the joists.
Cheers, Wayne
