I would put them in every bay, cheap insurance for a couple of problems. Imagine how much fun it would be to have to add them in later, if you started have problems with ice dams or similar problems.

Ice dams are caused by heat escaping through the entire roof and water from melted snow freezing as it passes beyond the insulation at the eave. To reduce this problem it is necessary to get the insulation as far into the eave space as possible and the baffles allow this to happen without blocking the vents.

The eave air vents are for the purpose of helping excess moisture to escape from the attic at the ridge vent. The amount of air flow through the eave vents is unlikely to be enough to have a substantial effect on the temperature of the roof so put the baffles where they will allow the most air movement. The foam type are vary good. Also be sure their net clear vent opening equals that of the ridge vent.

Thanks for the responses. I went ahead and put them between every rafter. Not fun work, but glad it's done!

want to come do a baffle install for me?
I'll gladly sub it out...LOL!

My ceiling was insulated with batts, and the baffles they used are only just enough to cover the existing insulation. If I want to add blown-in, can I just slide the longer baffles under the existing, or do I have to remove the existing ones?

It's a pain to have to do either, but if I don't, the blown-in will end up in the soffits.