http://www.boulderschoolofmusic.com/RhythmsFills.htm

This may help you. Basically, you need to learn all(or atleast some) of the defined rhythm patterns. The above page is usefull because you can get audio examples. Learning these defined rhythms can help you see what strumming pattern to play as most patterns are noted based on defined patterns such as waltz, shuffle, bosa nova, etc. This is what you would learn if you were to take percussion. Something not necessarily taught in guitar.