The only use I have ever made of the version number was so that a customer could tell me they're using version 2.5.1.0 or whatever.
My only rule is designed to minimize mistakes in reporting that number: all four numbers have to be 1 digit only.
1.1.2.3
is ok, but
1.0.1.23
is not. Customers are likely to report both numbers (verbally, at least) as "one-one-two-three".
Auto-incrementing build numbers often results in version numbers like
1.0.1.12537
which doesn't really help, either.
