Depending on the logo, it's sometimes better to convert them to bitmap mode rather than grayscale. Although this sometimes looks bad on screen, it can really help when it goes to print. However, it is dependent on the logo having flat colors, and being of a reasonably high resolution to begin with. If you don't have a vector version of the logo, it's definitely worth trying.
I did a newspaper ad years ago, and I could only get a tif version of an amex logo. I thought the best thing to do would be to convert it to grayscale, and color it in Quark. However, it was so small that the line screen made the logo completely unrecognisable. Converting it to bitmap solved the problem, because the line screen restricts itself to just the black areas rather than the entire image. It also had the advantage of allowing for transparency.
Using spot channels is a really good way of working, but I have found that some printers have trouble with it, so check with them first.