I've heard all kinds of things.
the 'X amount' of deflection is common.
I've been told that newbies are afraid to overtighten, but shouldn't be (in other words, tighten the heck out of it...)
I've also been told that bar-tight is no good.
All of these things are subjective.
My advice? Google search 'rig tuning' for your boat, and spend some money on a Loos gauge.
I have one - it takes all the guess work out of it. I may not have the rig
tuned quite right, but I know for a fact it's consistent year to year, and shroud to shroud.
(on my C&C 35, I use 1000lbs for the lowers, and 1800lbs for uppers)