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Cisco devices usually ship with a recent IOS version, which allows you to boot the device so you can load the IOS version of your choice.
As to best policy for IOS version selection, first you want a version that offers the features you MUST have. Second you might want to avoid old versions that are no longer being maintained (for the reason John noted). Third you might want to determine if a version/release has additional stability indication (the old GD or newer MD) and/or additional testing certification (safe harbor). Fourth you'll probably want the latest patch release for your selected version/release (unless it was just released unless if fixes a critical bug). Fifth (as also noted by Leo) carefully read release notes for any known problems for any thing that might apply to your usage (i.e. any show stoppers). Lastly, if possible, first deploy selectively and in not any essential parts of your network. Also deploy new IOS versions incrementally monitoring for any problems.
PS:
Regarding Leo's suggestion of "testing". In theory we all have a lab with the means to emulate our production environments. In practice, and unfortunately, I've found such lab capabilities to be rare. That's why I'm suggesting selective and incremental new IOS deployment. Basically it's (careful) "testing" in production.
