By wiggle room I'm assuming you mean working near or past the recommended specs.
The Max-Load specs should not be exceeded. Doing so might be okay in one gun, but not another, and there-in lies your problem: You don't know unless you try it. All you have is the SAAMI standards, which are the pressure tested loads in a limited selection of barrels, and results in an arguably arbitrary safety level set. However, they've been doing it for the better part of the last century in one form or another, so their info is likely the most accurate regarding safety.
So long as you're cartridge is loaded to less than spec in one category or another, your pressure will not be max. An example is a 155gr XTP in 40S&W, loaded on the Max charge of 6.0gr of HP38 at 1.125"
According to the Lee book I pulled it out of, this results in 1103fps at 33000psi. (BTW, Lee's data isn't their own, and has been show to be conservative in some loads, so checking several data lists is best)
Now, if I have any inconsistancies in my charge measurements or the seating depth, that load could be made higher than Max, so it's important to know how your powder dispenser and seating die perform with the components when working near max.
You can, however, give yourself some lee-way on this by changing the values. If your gun doesn't feed right with the listed OAL, then back off the charge some and seat a little deeper. Caution is required, obviously. Deeper seating decreases the space in the case and can affect the pressure increase more than reducing your charge decreases it. It's a delicate balance, but one that can be beneficial at times.
