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Manufacturers universally tell you to refrigerate mayonnaise once opened, though, because it helps preserve the flavor. Warmth causes mayonnaise to break down after a while, and it gets to the point where it just tastes oily. Homemade mayonnaise, of course, is a different matter. With its use of raw eggs, it is highly perishable and should be consumed immediately or refrigerated and finished off very shortly.
Commercial mayonnaise contains pasteurized eggs, as well as vinegar and salt or lemon juice. These ingredients acidify and preserve mayonnaise, so it's actually a very safe product. Of all the ingredients in your sandwich, mayonnaise is probably the least problematic one, unless you're using homemade mayo made with raw eggs.