Wow - fun thread.
First of all, I waited tables for 8 years - high school and college. Now I have a "big boy" job. They both suck.
Anyway, yes, you should tip for a growler fill. Why? Because that is how the restaurant is set up. It may seem stupid to the customer that you need to throw an additional $2 at someone to fill an eight dollar growler when the only way you can get that growler is by asking them to fill it. However, it is equally stupid that the bar tenders are only paid $4/hour by the restaurant under the guise that they will be tipped for everything they do. Their total income assumes being tipped, so it really is a responsibility of the patrons to make that happen. You got your growler, so the bartender did their part. Now it is up to you to do your part and throw him a couple bucks. Sure, quality of service comes into play, but it should really be a deciding factor between a 15% or 25% tip, not a factor between tip or no tip. I have to agree with what has been said - no one who has ever worked a day in the restaurant industry would ever consider stiffing their waiter/bartender/bell boy/maid/valet - whatever - for a service.
Don't blame the person filling the growler, blame the person who is underpaying his staff. Everyone else is just falling in line, and to take a stance of "why would I tip for something so simple?" really bucks the system.
On the other hand, travel the world a bit, and visit places where people in the service industry are paid a reasonable salary by their employer, and the tips are literally just a bonus. Guess what - the service (on the whole) pales in comparison to what we in America would actually consider "bad service". So is "assumed tipping" really a bad thing?
That said, when I get table service at a brew pub, I always settle the dining bill, and on my way out, swing by the bar for my growler. Watch someone fill that thing, and you will see what a PITA it is. I always get the impression that the bartender really appreciates me doing it this way, because the tip goes right into their bucket, rather than just getting a back end % from the servers sales. Sure, they already poured pints that were delivered to my table and settled as part of the dining bill, but again, that few seconds of work will go back to them when the wait staff has to "tip-out" for the night. In the case of a growler, it is several minutes of work, and I really think should be tipped directly.
So again, at least a $2 tip on a growler, and at least $1 for every drink across the bar. A little off topic, but I found tipping every time you get served a drink will get you better service throughout the night because it eliminates the mystery of you snubbing the bar after running a big tab. You won't get ignored if you run a tab, but you certainly won't get serviced before the guy who tips each time he got handed a drink.
Joe