Tax value for a gift of stock is not dependent on cashing in the
certificates. The value is set by the market value the date of the gift.
If you hold the stock and the value falls, the organization is out, not
the donor.

If the stock is a stock you would have in your investment portfolio,
then hold it. This may be for reasons of dividend income or the promise
of increased value of the principal - increase in stock price. I agree
with another comment. You should have a financial advisor who manages
the portfolio and answers to your finance committee on that portfolio's
performance.

A restriction on a gift is for use of the value of the gift. Most
organizations would not accept restrictions over their choice ot hold or
sell the stock. Once the stock is transferred and the organization
becomes the owner, the donor does not have much to say about it.