A time signature is a convenience for the reader--it just tells you how many beats to count to the measure and what the note value is for each beat--above and beyond that, there are conventions as to which time signature goes with which(as folks have mentioned above), but people break them or "re-invent" them all the time, both intentionally and unintentionally--It is perfectly possible to write a waltz in a 4/4 time signature(and it is done occasionally)--

The guideline generally has to do with this: every melody has an underlying beat or pulse(sometimes played outright, sometimes just implied)--and it also has a typical rhythmic phrase which also sometimes played outright, sometimes implied. The time signature depends on how many underlying beats it takes to complete the rhythmic phrase.

For instance, if the rhythmic phrase is: deedledeedle baba deedledeedle baba, 2/4 is probably a good choice, If it is Deedledeedle deedledeedle deedle baba, then 4/4 is probably a good choice--
(sorry for getting technical here)

It often is very murky though, because a lot of music is polyrhythmic, which is to say, there are a number of parts, each with a different pulse--one part may go "BumBum BumBum" another may go"buttabuttaButta ButtaButta ButtaButta ButtaButta" and another "ShoopShoop Shoobadoo"--In which case you just pick the time signature that is easiest to read, which is what is really about in the first place--