This is my second contribution to this thread. I keep getting the updates, and it's a little hard for me to see the basis for some of this discussion.
1. While it's not customary to tip in some countries it is here, so....it's customary.
2. It's become more than a custom, because employers and the IRS have gotten involved in it, and made the lives of underpaid service people harder than they were before. (No, I have never worked in a job connected with being tipped)
3. It's now to the point where we must acknowledge that tips constitute a significant part of the income of service people. Tipping is, therefore, and to whatever degree, obligatory if not mandatory.
4. My conclusion: YES. Growler, whatever, you tip. I don't see how filling a growler is any differnent than drawing a pint, and I tip a buck a drink, so there.
NB: Just to keep the record completely straight, I have never actually purchased a growler. We've got a couple of micros in the area, but they don't do growlers. You mention growler to people around here, they think you've got a mean dog.