I don't seam seal. I use a spray silicone waterproofing agent and get it real close to the seams and saturate it. I think seam sealer is messy and not particularly effective. I have done this with tents and tarps very successfully.
I do treat my claytor diamond tarp every other year with the spray waterproofing. I usually buy up some in late fall when it is at deep discount in dept. stores.
May not be necessary, but gives me peace of mind.
The World is my country, all mankind are my brethren, and to do good is my religion.