If a heater touches stuff, it usually doesn't cause problems, but aquarium heaters will destroy themselves if run in the open air because they are designed to be immersed in water.
Ergo, if you allow it contact equipment in such a way that they are not sufficiently cooled, they can distort and/or melt plastic surfaces, etc.
The smaller heaters don't cause as big of a problem because they just don't get as hot... if you are heating a 12g, I assume that the heater is give or take 50w... this should be okay, but I wouldnt press it against a plastic surface that contacts along the entire length of the heating coil area. It's the big boys... 300 watt, 500 watt, etc that really can cause issues.
In other words... touching is probably okay in your case, but laying against I'd try to avoid.