I see no issue with trimming a betta's (or most other fish) if it is for a health reason. Sometimes the only way to get ahead of a bad bacterial infection is to cut away the infected fin to give it a chance to heal. Bettas in particular can have issues with their fins getting so large that they can not wrap a female when breeding or can have difficulty swimming. All 3 of these issues (to me) are reasons that a fin can be trimmed. Trimming for cosmetic reasons is both unnecessary and needlessly stressful on the fish. Trimming a fish that will be shown to get the perfect look is just downright cheating.
The benefit when trimming for the right reasons can outweigh the stress on the fish while the procedure is being done. Some situations can be life or death if the infection is bad enough. I have had to do procedures like this and never had an issue from it. As long as the fins are not cut too close to the body, there are no blood vessels and no nerve endings, it is like clipping nails (yourself or other animal).
This boy is one I had that is called an extreme rosetail. His fins were so branched and frilly that he could hardly swim most of the time. He stayed near the bottom or resting on a plant higher in the tank except on the occasions he decided to move. As seen in the picture above, even in full flare his dorsal fin and caudal fin are not able to fully extend.
In this next picture it shows him more at rest like he was normally on the substrate. Notice the dorsal fin hanging over the side of his body and his caudal all bunched up.
Another view of him in a normal state. Since he is up off the substrate this picture shows how his body seems to tilt down from the weight of his fins.
If I had known about trimming fins when I had this male, I would have trimmed his fins. He would have really benefited from the trim and ability to swim normally. As it was he passed earlier on than he probably "should" have and as he got older battled rough cases of fin melt/rot which he could not fight off like he could when younger.
OP -- In all reality, trimming does not ensure that the fin will grow back like it was supposed to. Bettas that have had fin damage can have the new fin grow back in quite funky and repeat offenders(tailbiters) -- there really is no way to tell how theirs will turn out. So I don't think that unless your boy has an issue with the way his tail is, that trimming it would do you much good.
And just because I ran across this picture which clearly demonstrates how easily a betta can get to the end of his tail if he wanted to.
Note: this male was never a tailbiter, although his fins look like he was. His fins naturally had that weird uneven look from having a bit of rosetail gene in him.