+1 AROTC-Dad
IMHO I think it can also depend on your Congressional District. For example if you are in Northern Virginia Congressional Districts 10 or 11, where 200-300 candidates apply each year with each MOC for one of the 10 nominations, and the MOCs have a good process to deal with the volume, such a letter may not really matter.
However if you are in a Congressional District that isn’t as large or competitive, a respectful humble sincere email or letter may be a good thing to send. It is not necessary, but it can show initiative, gets a file started, and, as has already been mentioned, may get you put on a mailing list for SA events.
When I applied to USAFA ~40 years ago, I actually sent my letter a year early, in my sophomore year. I got a nice letter back from my MOCs letting me know I had expressed interest a year early (I knew that) but it got my file started with each of them. I then sent them articles from the school newspaper and the local paper when I was mentioned in school clubs and school sports. I think this worked for me because I was applying from a “rural” state. I got all 3 nominations, but didn’t win any of the 3 slates, but did get named as a #2 on one of the slates. Did my letter do it? Don’t know ... but it didn’t hurt.