I used a tank that came off a bike that had been off the road for 12 or so years. I cleaned the petcock completely spotless before I ran the bike and did a couple of rinses with fresh gas. Then ran Seafoam in the first few tanks. I've been running the bike for 2 months now.
Yesterday I did a tank swap and when I pulled the petcock off the old tank this is what I found(plus a little more that didn't make it into the picture). The bowl was completely full. I assume this is the remains of the old gas that dried in the tank over that 12 years.
So if you're breaking in a tank that has been sitting for a while, keep an eye on your petcock. I can only imagine I'll need to go back through my carbs now too.