For the most part, songs are usually played using alternate (up and down) strumming. But there isn't any hard and fast rule. Usually what I do is just go with the flow. Occasionally I'll mix it up and do just down strokes, or palm muting, soft strokes hitting just specific strings etc...

Have a look through the Sevenths chords series I did.

http://guitartricks.com/lesson.php?input=12424

There's about 14 of them at varying BPM's and styles.

It might not be your style of music but the videos will give you a good indication of how I vary my strumming techniques as I'm playing.

Or try the minor chord series I'm working on now:

http://guitartricks.com/lesson.php?input=12655
http://guitartricks.com/lesson.php?input=12656
http://guitartricks.com/lesson.php?input=12657
http://guitartricks.com/lesson.php?input=12658

Each lesson has an MP3 as well as a Jam file to practice to as well so you can study the video and then try the techniques yourself.