They're really not strict, but it's always gonna be YMMV. The problem you'll find is that on a regional jet they are a bit more anal just cuz stuff doesn't fit even when it is lower than the normal size. The solution is, if your bag is close "carry it on" then when you get to the gate you'll find either A) it fits B) there might be skycheck (others will know better, it seems hit or miss to me as to when they have it vs when they don't) or C) they'll just check it for free, not ideal since I'm assuming you'd rather carry on, but at least they won't charge ya.