If you have a perfectly square easel (and assuming the paper base matches the matboard pretty well) then it wouldn't be a problem to leave a channel with a little white boarder showing around the edge of the image. When mounting my platinum prints I make archival paper corners that I tape to the underboard with linen or tyvek tape because the self adhesive plastic ones are generally too small and flimsy. If you print right up to the edge then you will need to use something like the clear view photo corners. I would only suggest going with a slightly smaller channel—1/4-inch on the top and sides, and 3/8-inch at the bottom, especially if it is an 8x10-16x20" print. I have seen a lot of prints mounted with larger channels and the proportion to the print always feels a little off.
One of the main reasons for drymounting, and why Michael Smith and Paula Chamlee do so, is because single weight papers like Azo are impossible to keep flat and will crimp if you so much as look at it wrong. Drymounting to a larger underboard is the best way to keep the print surface and edges safe when handling and displaying. The other reason is that even if the black edges were not trimmed from the contact prints the window would need to overlap too much of the image too keep it completely under the opening.