Should be fine as .psd is the native file format for Photoshop... you will have brush issues as you can't use CS4 brushes in CS3 (or any downward versions...) but if i do remember there was an option to "maximizing compatibility..." in the preferences. this is to allow the layered PSD file be used in other programs and earlier versions of PS.

i never really had to use it but if you want to try saving both methods and see if you have any problems.

bear in mind, saving with the "compatibility" option increases your file size by a bit.

hope it helps,
scott