I recommend not telling about the cert in advance - it may reduce your upgrade potential if they know that you will be spending less than other Ambassadors staying at the same time. Although this won't be a problem with the Presidential Suite ....!

I have used certs for suites, albeit not top-end suites, on numerous occasions and have never had a problem. In fact, I find this the best benefit of Ambassador since I like to guarantee a big room when I travel and the voucher usually makes the price very reasonable.