At the start of a pz2 file it says:
version
    {
    number 7
    }

Older pz2 files says "number 5".
If a product is to work with P5 and P6 do I need to change the version number (to 5) or can I leave it at 7?

Generally the older versions will still load the file but a rather annoying error message will popup telling you that Poser doesn't recognize the version number but will attempt to load the file anyway. 

As I said, generally it will load, but if you're going to offer the file to others it's a bit more considerate to change the version number just to spare potential users the annoyance of an error message.  And possibly spare yourself an onslought of irrate e-mails from annoyed customers complaining about it..

Thank you SYNTRIFID, good advice. I will change it to a "5".
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I generally change the version number to 4.03 so it will load without the error message no matter what current version of Poser is being used. Not everyone uses 5,6 or 7 As SYNTRIFID said, it is just a consideration, not mandatory

Good point nickedshield, but this product will only work with versions that handles dynamic hair. If I remember right it starts with P5. Or was it P4?
hmm... now I'm confused. Anyone remember?

P5 was the first Poser with dynamic hair.

Thanks karen1573. Glad I'm not all senil yet....