I found the questions relating to polarizers, but haven't found any info on a 300mm zoom lens with a polarizer. Is this because it is not suggested to be used on 300mm zoom lenses? I use this lens to do close-ups indoors and outdoors with a tripod. I know there is a trial and error factor, but I was wondering if I'm wasting my time with a polarizer that takes off approximately '2' stops on a 70-300mm Sigma Macro. The question pertains to the 300mm setting. Sorry if this has been answered somewhere here, I read through a bunch of Polarizer-tagged questions and came up blank on a 300mm lens+ polarizer question.
but haven't found any info on a 300mm zoom lens with a polarizer. Is this because it is not suggested to be used on 300mm zoom lenses?
No, I think it's just that there's nothing to say about that specific focal length with a polarizing filter -- it'll work the same as it does at 200mm or 100mm.
A polarizer's effect depends on the angle at which light is being reflected -- mainly, the angle of the sun. Since wide angle lenses capture light coming from a large range of angles, circular polarizers can cause a blotchy sky with wide angle lenses. But with a telephoto lens, particularly one as long as 300mm, you won't have that problem.
In what type of conditions do I use a polarizer on a 300mm lens?
Use one when you want the effect that it gives: reduced glare, deeper blue sky, rich foliage, etc.
There are polarizers for long focal lengths, but you usually find them in the shape of circular polarizers : http://www.the-digital-picture.com/Reviews/Canon-52mm-Drop-In-Circular-Polarizer-Filter.aspx
You also have less chance with a long focal to have a dark line in the image, as seen here : http://havecamerawilltravel.com/photographer/polarizing-filter-wideangle-lens
Using a filter on the front end of a telephoto lens requires a better grade of filter than one needed for a wide angle lens.
Consider: A telephoto is taking a narrow view. A small angle of the world is smeared out over the entire sensor. Any flaws in the filter are similarly magnified.
The article Evaluating Filter Quality at clarkvision.com goes into detail. Even a good filter on a long telephoto can be troublesome.