Yes, either merch table or wait before/after the show.
My first meeting with Johan was at random. I just happened to be driving into Philly when he walked out of a Chinese resturant and in front of my car.
We kept driving and found a good parking spot, watched the show, but we missed half their set because we were at a Children of Bodom signing. During COB's set, I went back to Amon's merch table and they were there signing autographs. I hung out with them for a while, got pictures and stuff signed, and did it without annoyance.
A few days later I went to New York to see the same show, and hopefully see what I missed from Amon's set. The show was sold out for two months, but my brother and I ended up getting tickets that very day. Talk about luck,hahah!
Anyway, we started walking down the street, and as soon as we turn around, we run into Amon Amarth. They took me and my brother out to lunch with them, it was awesome.
Oh and Johan loves my mom
Moral of the story is, they're awesome guys, and they won't say no to an autograph or picture. Come before a show, stay afterwards, they'll come out, be patient is all.