I agree with James's answer, but I wanted to work out the specific figures.
- Spawners are active within 16 blocks (Euclidean) of the player, and may spawn monsters up to 5 blocks (Chebyshev) of themselves [ref], for an absolute maximum player-monster distance of ∼23.07 blocks.
- The exclusion radius around the player is 24 [ref].
So, from these figures, you cannot have other mobs spawning as long as you are actually at the spawner.
- If the chamber around the spawner has dry ground in a larger area (by two blocks) than the spawner's area, then it could spawn other mobs. Therefore, don't exceed the 8×8 (or 9×9 if you don't like pondering coordinate axes) optimal size.
- As James pointed out, if the spaces connected to your idling location are not perfectly lit, something could wander in from there; therefore if you don't trust your lighting skills you may wish to put doors around your idling location.
These conclusions agree with my own experience: I have had zombies and creepers in my skeleton farm (my design has some dry ground in the spawner chamber), but only ones which spawned before I arrived at it — never any more once I start idling at the output.
You specifically mention being worried about creepers. One way to fully protect yourself is to make sure that the mobs do not have a line of sight to you — creepers will not explode and skeletons will not shoot unless they can see your face (and not even through glass). Therefore, a simple safety measure is to confine the mobs such that blocks are in front of their faces and you hit them in the legs. (This doesn't work in SMP due to bugs.)
Also, you can protect against spiders in your zombie/skeleton trap, or creepers in your spider trap, by making sure there is a passage only large enough for that type of mob.