Present vs. Future Tense: Appreciate

Hello,

Is it proper to use the present tense in the following sentences at the end of one's request in an email?

A. Your assistance is highly appreciated.
B. I really appreciate your assistance.

I feel the future tense is more appropriate and common in sending requests, like:

C. Your assistance will be highly appreciated.
D. I will really appreciate your assistance.

However, I believe A and B are more appropriate when a request has been granted. Would you agree?