I think it is ok to send a standard letter but consider....
When you receive what appears to be a form letter how do you feel? or if you get a letter where a coach mentions something like I was impressed seeing you at so and so tournament when you struck out the side or got the big hit... If you received a letter like that you would be beside yourself with joy even though you might be happy with the standard form letter as well.
I don't believe in doing things just for the sake of doing them. If you can personalize your letters in any way then I believe in doing so for it shows initiative. Could be anything like I have been a UNC fan my whole life... or my coach said that your camps provide some of the best instruction in the country, or I was really impressed with how your team performed in the NCAA regionals last year etc.
Do things with a purpose and put your heart into it no matter what the activity and you might be surprised at the results