Depending upon how the covering material you're using will stick to itself, you should have no problems doing the tip first, and then the rest of the wing.

The usual patter of "bottom, then top" should be followed. This allows the seams at the leading and trailing edges to be less visible to the eye.

Use the end rib's inner surface to give you some extra surface to enable you to get a good stretch on the covering when you apply it.

Contenders have had upswept wingtips in Central Illinois since the 70's, with many, many of them covered in MonoKote. Of course, that's what we use on our models!