With panniers you really do have to be careful when you load them..... when I was touring I had to move a pair of sandals from the left to the right pannier as I tipped over twice in a row at stops from being just a tiny bit overbalanced on my clipped in side...

It's highly unlikely that you will ever break a bike with weight.... My husband did break some spokes once, but he's a big guy (he's 6'4" and I think 210 at the time) *and* we were fully loaded for camping. At your weight, even with as many groceries as you can carry, I don't think you could really break a bike, so don't sweat it. As long as you keep your tire pressure good you shouldn't flat... that's usually only a problem if you get low and pinch flat.