Where is the most important part to sound deaden?

I took out most of my interior on my project car to get it dyed, and now I think I will be working for a traveling company again meaning I'll be driving up to 10-15 miles straight on the interstate several times a week.
The car is very loud right now, but I know that is because of the interior not being in. But I went and bought some peel and seal, saw a buddy use it and it worked great for the price, and was wondering where the best places to put it is. I was wanting to put it on the floor from the hatch door to the front seats, I do not have back seats for traveling, but my hatch leaks in the back and I want to get that solved first.
Would it be a problem to place this stuff on the inside of all of the interior panels?
The car is very loud right now, but I know that is because of the interior not being in. But I went and bought some peel and seal, saw a buddy use it and it worked great for the price, and was wondering where the best places to put it is. I was wanting to put it on the floor from the hatch door to the front seats, I do not have back seats for traveling, but my hatch leaks in the back and I want to get that solved first.
Would it be a problem to place this stuff on the inside of all of the interior panels?