The best venting for a roof is at the eaves (bottom of the roof) and ridge (top). This venting should be above the insulation and should not allow bugs inside.
Here is one product we have had good luck with. They make versions of this corrugated plastic vent strip to fit all kinds of situations.
(http://www.cor-a-vent.com/images/V600SHINGLES.jpg)
http://www.cor-a-vent.com/products.htm
Note: Please add you suggestions as a follow-up.
For metal roofing you just assume it will vent because of the various corrugations, I gather. But what if anything can you do about bugs? Don't worry if it's over sheathing, you get what you deserve if it's not?
Not important to me now--the metal roof on the barn is on purlins, but the attic area is more or less open on both ends.
Manufacturers of nearly all metal roofing have die cut rubber plugs in strips usually 3' long to fit both the inside and outside configuration of their sheeting plugging all holes. There are lots of slightly different designs.
Aha, there seems to be a special Cor-a-vent (possibly Fold-a-Vent as well) for metal roofs--that doesn't have to be shaped to the roof.. And the shaped foam pieces go under that. But all I'd ever seen (and all that was on one metal roof company's web site) was the same old picture of the asphalt setup.
In fact I've noticed those extra tall roof peak covers on a lot of houses in the area.
e.g. here
http://www.englertinc.com/phase-2/standing-s/accessor/ss-access-1.html