JCW,
Either or. There are 2 ways to approach this. You can find differences at the same time - sometimes with opinion, sometimes with inflation, sometimes with stale lines. This is the squarer and less risky of the two. The second means is to take a lead - maybe you're sharp as hell, or have early injuries or weather status. Of course, you buy back after the dust settles. The only probably with this is that you often sacrifice a bargain (weather, injury) for the sake of middling. It's 6 of one, half a dozen of the other.
K,
Yes, IF you play the right numbers. Don't expect to make money middling 1/2 in football - you'll go broke VERY fast. I'd recommend building/aquiring a few seasons of boxscores and really putting the time to understand the expected ROI on middles.
J loq,
Yes you get 10-1 on a side, 20-1 on a middle IF you play 20 cents. There are enough reduced juice and opinionated shops to reduce this juice dramatically.
For instance, I hit a 63-1 side on a NBA sprear earlier this year. These are extremely rare to find, but they do exist once in a blue moon.