If it is nylon or polyester, no need. These fabrics are coated on the inside with polyurethane. The spray on waterproofing will not help and might actually cause the polyurethane to delaminate over time. (Don't ask me how I know this!).
If the tent is canvas, I'd look for a special canvas water proofer. Canvak may be one.
Heres a tip to help eliminate leaks.
Don't fold your tent, stuff it. This way any creases will be random and not break down in the common areas. Depending on use, seam seal your tent every year (we camp several times a year but have different tents for different seasons so its every other year for us).
+1 for stuffing the tent and using seam sealer. McNett makes a good one that you just brush on the inside of the seams and let dry overnight. Super easy to use. I used it on a tent I bought about 6 years ago and have since slept in it for about 60+ nights with all kinds of wet weather and have never had a leak. I wouldn't throw your tent in a machine tho, I've always just hosed my out in the backyard if it gets too dirty. Nixwax is usually used to restore waterproofing and breathability on gortex-type items, not provide initial protection.
Has anyone ever used never-wet for anything as serious as a tent? I'd be curious, as my only experience was just kinda messing around with it on random stuff with mixed results. My worry is that it require frequent reapplication on a tent, but maybe I'm wrong.
I've sealed seams and water proofed home made rip stop nylon tarp/poncho with mineral spirits and clear silicon. Mix silicon into mineral spirits until dissolved and brush on seams or for large pieces I submerge in a bucket, let soak then lay out on line to dry. Cheap and works great.