I once read the tech details on rehashing and found it a bit dull, so i purged it from memory. The thing to remember is that if you ever go from a higher number firmware to lower, than you need to run "toggle qa" because the older shiznit might not be compatible and such. Bottomline, it is really easy to run toggle_qa. pkg so just do it. I did read that if you have done it once, than the seed hash is set, and therefore you never need to do it again, and it doesnt hurt to rerun it many times.
I see people recommending turning QA flag on. It's to dehash... what exactly is dehashing? Read that it's to be able to downgrade again to 3. 55?