I would avoid using a regular heat pad from the drug store. You can't apply very much weight on them, they tend to short out. You could end up with a surge that could heat up your tank or jump a spark in your wall outlet.
::Well if the temp is cold and the substrate is a little moist then it would be a good idea to warm it up a little. Some of those heat pad from the drug store I think have temperature setting; you don't want to set it too high.