Gregory S. Forman, a lawyer from South Carolina, has a helpful blog post titled "What makes a good request for admission?" If you happen to be drafting some requests for admissions, you might want to take a look. I advise reading the whole thing, so that you'll fully understand Forman's closing lines--
Thus, the ideal request to admit is: 1) not trivial; 2) not already acknowledged; and 3) narrow enough that an admission is useful but a denial is subject to impeachment.
If you regularly use requests for admissions in your cases, I also advise you take a look at a tip I wrote about at Trial Practice Tips some years ago: "Discovery Tip: Interrogatories About Requests for Admissions."