+1, 91 will do nothing for performance unless you are tuned for it. Can actually hurt rwhp and if you have cats, the unburned gas can actually crap them out.
If you are stock, don't even both with a tune and 87 is fine. When you start buying/installing mods, then I would call a local mustang tuning shop around your area or maybe go with www.americanmuscle.com
and buy their Xcal3 with a bama tune. Since you are N/A, won't be as hard to do a email tune vs if you had a power adder.
Also, 87 has a better burn rate than 91 or 93. Why some people think it's good to give their car "some of the good quality stuff" is beyond me. Now the S197's with VVT, it's been dyno proven that if you go from 87 to 93, you will gain some rwhp. But 2004 and older...no.