The T5 trannys put into the 1992 and up model year were improved/upgraded to eliminate this reverse "problem", as well as a 3-4 sychro issue:
The following is a direct quote from an excellent and quite thorough article on the full history of the T5 (Non-World Class and Word Class) titled "World Class Act" that was written by M. Davis and appeared in the May 1992 issue of Car Craft magazine:
"On Ford 1992 Mustang V8 WC (world class) transmissions, the organic 3-4 sychro blocker ring lining was enhanced for greater durability. Previously, the material's effectiveness could degrade if it was subjected to missed shifts or clutchless shifts. The latest compound can tolerate much more abuse...and can be retrofitted to and T5 WC trans...also new on the '92 Mustang is enhanced reverse engagement. After coming to a stop, it isn't necessary to wait to engage reverse, or 'clear' reverse gear, in order to avoid gear clash. Moving the shift handle/linkage stops everything in the box. This design change cannot be retrofitted to earlier transmissions."
I personally lucked out, in a sense, because my 1991 GT was actually made in the late spring of 1991 and the original owner before me had a tranny issue in 1992 while the full warranty was still in place. Ford was interested in taking the original tranny back to investigate what went wrong and so installed a complete new one in its place (a 1992 T5).
THAT BEING SAID I have driven both types of T5s and can attest to the fact that the '92 upgrade really works. However, when driving a T5 not equipped with this feature, I would always just hold the clutch all the way down and give it 4-5 seconds to stop spinning inside, after which I would robustly slide the stick into R. If I didn't feel like waiting, then I'd slip it into 1 (which would stop the internals from spinning) and then into R. The trick is to not try to engage R while the internals are still turning.
ADDITIONALLY, in my '91, even though it has the reverse gear upgrade, every so often, maybe 1 in 15 or 1 in 20 times when I try to put R in, the gear will simply not go in at all (no grinding, just won't go in). It seems that if the internals happen to line up just right way the R "teeth" will not mesh. All I do at this point is to slide the stick into 1, which always nudges the internals a bit, and the R goes in perfectly fine, every time.
I love driving my T5 - really well made and shifts like a dream - what a machine!