one large benefit of saving in psd, at least in paint shop pro 7.xx, is that it saves everything, layers, command history, masks, channels, everything. this is the only format in psp 7 that does this that i'm currently aware of. i'm sure it must be true in p-shop also. the one big downside is file size and that makes loading and saving slower. but for saving to work on a piece later, psd is the one to use.
for pure archival purposes, .tif or .bmp would be my current choices. both can easily save in high resolution with no compression. .bmp has the added advantage of being highly portable and highly used and very acceptable on almost any web site where some sites wont display .tifs.
i also took a look at the .tif format in psp. it doesnt save layers for me.... or maybe i've missed an options setting. i'll look again.