Why would you need to seam-seal it - my P.A. 1 has taped seams. Also IIRC the outer is P.U. treated, so it doesn't need proofing. (BTW, isn't Grangers only for cotton tents?).
Jim
In my previous experience, taping on cheaper tents is not very robust. For the sake of an afternoon with a paintbrush, seamsealer, and tent UV/Water proofer, you can significantly improve the life and performance of just about any tent!
Seams are generally under a lot of tension, particularly in the wind. If a small amount starts to unpeel, the water and tension work together to strip the seam tape. One could argue that it's a bit 'belt and braces' but I sleep better better with two lines of defence!
The UV/Water proofer will help extend the life of the fly material. Plastics will degrade in sunlight due to UV radiation which will also 'bleach' the material. After use in strong sunlight, you can sometimes see the PU coating start to peel away from areas that rub or are under tension (like skin peeling after you get sunburnt). I'm certainly not telling people to go out and do it; it's just good practice if you want tents to last for a maximum period.
Both Grangers and Nikwax do reproofing and UV protection products - you can get them for cotton canvas tents and also for synthetic fibres.