Re: What version to use for a new project?
No do not start with 1.0 unless you plan to not use any plugins.
You can always upgrade from 0.9 to 1.0
You can *never* downgrade from 1.0 to 0.9 because of the dramatic db changes involving tags and meta.
Even if you are “only using part of bbPress” (not sure what that means) 1.0a6 is about to do a semi-radical change with 1.0a7 involving permalinks.
bbPress 0.9 templates will (mostly) work with 1.0 but not visa versa.
1.0 is about 150% the code size than 0.9 for the same functionality.
I could go on but the point is that 1.0 is changing and unstable by nature. It should not even be called alpha but pre-alpha.