I just replaced the tank on my 170 about six months ago. There are many different opinions on this, but when I replaced my tank I didn't use any foam on around the tank. I just laid it in the tub and used a little weather stipping where the straps contact the tank. To me, this will really cut down on moisture.
To answer your question, I would cut out the foam especially if it is wet. I think that it would prolong the life of the tank.