The baseball community is small and they talk. We always responded to the letters, after he committed, we would send a email to the coaches who he talked to and let them know he appreciated the interest and he had committed.
You never know what is going to happen and you may want to keep your options open if it does not work out.
Some coaches may respond with congratulations, some not at all, and some may come back at you and let you know what a big mistake it is to go to XYZ, and others may throw at you the first time they see you in the box! jk