The thing about corners and edges is any number will only be in a maximum of 3 or 2 (respectively) lines out of the 8 possible, whereas the middle number will be in 4 (so that's half your lines right there). As such, I always try to go for the middle first, then corners for their 3 lines each (up/down, across and diagonal).

The only exceptions are when the exposed number is in the middle (haven't seen it yet, but if I ever do I'll just do corners probably) or if it's really really low and I'm hoping for a 6 (then I'll try a number next to it).