Rule I've been following for a while that seems to work pretty well is "No sag / No drag" - the chain should be JUST tight enough to be contacting the bar all the way around, but not so tight that there is significantly more drag than on a fully loose chain. Usually to get this, I loosen the chain with the bar adjust screw until there is a LOT of sag, then while holding up on the tip of the bar enough to lift the front of the saw off the ground, tighten up the chain until it just barely touches the bottom rails of the bar. Note that if you go to far, you need to back WAY off until you are back to the lots of sag point and start over.... If you pull the chain around when it's loose, and then try it again when you have it properly adjusted, the amount of drag should be the same, i.e. very little.
Excessive drag should be avoided, as it increases wear on the bar, all the chain rivets and the clutch/sprocket.... In addition, it increases the amount of saw horsepower that has to go into dragging the chain around the bar as opposed to cutting wood...
I have found that when adjusted to this level, the chain can be pulled away from the bar, but it will "snap" back when released - how far away from the bar the chain can be pulled depends on the length of the bar. On my 12" bar Pull-on I can hardly get much of a gap. On my Dolmar with the 20" bar, I get about half the drive link, on my 28" bar I get about 3/4 of it.
If this is done on a broken in chain, you should be good for quite a while - you may see a LITTLE sag when the chain is hot, but not more than about 1/2 a drive link, and this will disappear when the saw cools back down. I sharpen the chain on the saw, and find that I only need to adjust the tension every 4-5 times I use the saw.
If breaking in a new chain, you may have to adjust it a couple of times as it stretches out - however if you adjust a chain when it is hot, you should back off the adjustment before the saw cools down, especially on a big bar. According to the guys on Arboristsite, a chain that was adjusted when hot will shrink enough when it cools to possibly damage the bar mounts or other parts of the saw or even bend a crankshaft...
As to bar flipping, I do that any time that I have the bar off the saw for any reason, but usually I don't take the bar off just to flip it... (Also note that if you have an asymmetric bar like the "banana bars" they used to have on some of the older saws, you should not flip it. However the modern style symmetric bars should be flipped on a regular basis)
Gooserider