Generally, you can do them on the purl side. If you have to bind off any stitches, though, they have to be done at the beginning of the row.
If you'd rather do them on the knit side, look at your piece and think of your decreases as being on the neck edge or the sleeve edge.
If you did a k2tog on one side, and you decide to do the decreases on the knit side, do a ssk so the decreases will match up.
Is that clear enough?