I preffer not to tape the top of the tank since that just restricts the convection airflow, and the tank turns into a carbon dioxide bath tub for your snake which is dangerous. If you don't know CO2 is heavier than air and it sinks and accumulates therefore at the bottom of the fish tank.
I can't stress enough how fresh air in a fish tank is due to the fact that fishtanks are watertight enclosures unlike custom cages.
Any animal subjected to higher than normal atmospheric concentrations of CO2 can suffer many ill effects.
So side insulation is great. Covering the top is a big NO in my book. It will save you few cents on the electric bill but think what you're doing to the snake.

for more info http://en.wikipedia.org/wiki/Co2