It's funny, because I avoid cross-chaining, but think about it - does it really make sense to think that, on the other 9 sprockets, moving from high to low, things are just fine, fine, fine... until you click into that 10th cog, and suddenly it's BAD BAD BAD? Big and second-biggest can't be that much gentler on the chain than big-big. Modern 9 and 10-speed chains are really flexible. It's probably fine. I don't mostly because the resulting derailer rub drives me crazy.