Two tricks

(i) If you have a layer with a cutout in it (i.e you've already deleted and there are some transparent pixels) and it's got a pixel 'fringe', make a selection based on the layer's transparency (ctrl-click on the layer thumbnail) and add a layer mask (reveal selection). Don't really know why but this gets rid of the fringe (I do know why but i hate typing)

(ii) Don't delete pixels, use a layer mask. Run a mild gaussian blur on the layer mask then use levels on the layer mask to expand or contract by a few pixels (try it, it's easier to do than to explain). This is the only way to expand/contract a mask with full visual feedback

Oh one more... if you use two layers, not one, you can often cheat by contracting the cutout of the top version by enough to remove any fringe, then set the lower copy to multiply (if the fringe is white) or lighten (if it's black).

Just another route to the same solution...
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Simon Joinson Editor, Total Digital Photography, London UK
www.totaldp.com