Most shops I visit will overshoot by a couple hundred psi to give you an honest fill to the service pressure (or 10% overfill as appropriate for "+" rated tank) when the tank cools.
On the one hand I think any shop owner with brains understands the meaning of the CFR's and also realizes that tank pressure changes with temp and that consequently service pressure is dependent on temp and needs to be considered with the tank at room temp.
In short overfilling slightly to compensate for the tank being warmer than normal is an acceptable practice. The previously cited CFR reference that 3AL tanks can be filled to no more than their service pressure is a reference to the plus rating also cited in the CFR not being available for 3AL tanks, not a prohibition against filling warmer than normal 3AL tanks to pressures slightly and temporarily higher than the service pressure. After all, 3AL tanks sit in the sun and in car trunks as well, just like steel tanks.
A decade or so ago a company sold what amounted to a temperature indicating sticker for scuba tanks. It was designed to be stuck on the neck of an AL 80 and was intended to be used to assist the fill operator in determining exactly how much of an overfill would be needed to produce a tank with 3000 psi in it when it cooled to 70 degrees. I thought it was finally a very practical application of 1970's mood ring technology and a step in the right direction as it was more accurate than feeling the tank and estimating from that. But, I have not seen one in a long time so they obviously did not catch on. (I have not seen a mood ring in at least 20 years either.)
On the other hand, the federal government in general and the DOT in particular went through a period of over enforcement of it's regulations. I noticed this most in the actions of the Federal Aviation Administration who got overzealous in busting pilots for all sorts of minor unintentional minor infractions of the FAR's and even for things that were construed as infractions but were not by over zealous FAA inspectors (Bob Hoover getting jerked around comes to mind). But I have also heard stories from this smae period of dive shops being fined for overfillling by a couple hundred psi on a hot fill - something that would be legal per the regs provided the tank was at or below service pressure when it cooled to 70 degrees.
So even if those stories are an urban ledgend I can understand why some shops may be reluctant to do this even in the much more consumer freindly and relaxed regulatory enforcement climate that exists now.
My suggestion is that if you keep getting short fills, complain to the shop owner, spell out your arguments, tell all your dive buddies, and take your business elsewhere to hopefuly create an incentive for the shop owner to take a more reasonable view of the regs.
