it probably isn't too bad unless you're in full yankees gear I think in general non-"rivalry" interleague matchups are fairly tame in all parks. I would assume there is going to be a couple A-holes jealous of the Cubs being the newer story and wanting to lash out, but by and large it shouldn't be an issue. I haven't been to Fenway in years though. Can't speak to the Fenway question specifically, but was in Boston last week and heard a lot of "soooo happy you guys won, love Theo, also love Tito and would've still been happy if Cleveland won because they haven't won in a long time either. But really, really happy the Cubs won." etc. I'd agree that it shouldn't be too bad unless you go out of your way to be a jerk. For example, I wouldn't chant "the '04 Sawx were juicers."