Yes, without any problem!
CS5 (12.0.4) is the same as CS5.1 (12.1), the only difference being that 12.1 can be rented, 12.0.4 not: http://blogs.adobe.com/jnack/2011/05/photoshop-12-1-photoshop-12-0-4.html
But don't confuse Ps CS5 or CS5.1 (Photoshop 12) with Photoshop 5
There is always the preserve compatiblity check box to save to older versions that might not support smart objects, newer blending modes, etc.
Have a good week-end!
Pierre has provided the info for Photoshop Images, but since you mentioned "Production Premium," I wanted to point out that with some progrms in that suite, things might not work quite so well, when going "backward." PrPro (Premiere Pro the NLE video-editor) is an example of that. Going forward (up from one version to a later one) nearly always works well, but going from a newer version to an older version might have issues, if they work at all. In the case of PrPro, one is working not with Images (much easier), but with PRPROJ files, which are basically XML database files.
None of the above impacts Photoshop Images, just some other programs in the various suites, and is for information only.
Thanks, Bill! Yes, relevant info for the Suite.