I think the OP's question is whether he needs to brace the structure while he's working ?
There's a lot of structure already, so if the back glass, surrounding structure, and quarter panels are intact, bracing isn't necessarily required.
Easy enough to verify - and correct if necessary - by taking before & during measurements.
One could place a strut under the package tray/trunk lid lip area to the trunk floor if need be.
That could even be a wood 2x4 with a tight fit.