After a boil up in the queue for the Blackwall Tunnel one evening (almost 4 hours to do 1/2 mile!) I asked around what precautions to take before taking the car to Techniques for a proper inspection, and they recommended K-Seal very strongly.
Like Simon they use it in all their race prepared cars just as a precaution.
I take Richard's point about it being better without, but more modern formulations have largely ameliorated that one.
