There is a similar, more broad mantra which I have come to employ in some recent database work - Think about how you're actually going to use the data. Spending some time to address this question early in the database design process has allowed for some very significant improvements in some code which I am working on currently. In a similar fashion to demerphq, time stamp entries across a series of database tables in some code that I am working with is now stored in an unsigned integer as seconds since epoch - Furthermore, aided by the MySQL internal commands for address conversion, some network mapping database entries are now stored in long integer format and as such, some large portions of network address calculations in userspace have been able to be removed and replaced by internal MySQL address conversion functions. eg.
perl -le "print unpack'N', pack'B32', '00000000000000000000001000000000'"