Larger Filters on Smaller lenses with a Step Up ring, you're never going to have a problem, except if you want to use a hood. Sometimes you can get the hood on and *then* mount the filter, sometimes you can't even do that. But if you don't want a hood, there's no problem at all. (I'd recommend getting another lenscap, $2 on ebay, the size of your filter, so that you don't have to unscrew everything to put the original lenscap on).
Putting Smaller filters on Larger lenses with a Step Down ring is also possible, but not recommended, because you get worse vignetting. Only if you're putting an EF lens on an EF-S body, then maybe it's ok, but it's up for experimentation.