The ultimate goal is for the paper to flow smoothly. Therefore, my favorite style is to place some proofs right after the propostion, while some others in an appendix, depending on how much insight the reader gets from reading the proof.
If only the result of the proposition is important and the reader is better off reading the next paragrah and skipping the proof on the first read, then place the proof in the appendix. The reader can jump to the appendix now, but you're giving a hint that it's better done later.
But if the proof elicidates the material and you do want the reader to read it before going on to the next paragraph, then place it right after the proposition. A good practice I wish I saw more of is to have an outline of the proof after the proposition and the detailed version of the proof in the appendix.