The mods *do* read every entry before it posts. And we do basic proofreading as well. However, we are not a substitute for a beta. There is always a risk your recipient won't like their gift. It's unfortunate, but does happen. Some really very good fic sometimes just isn't someone's cuppa even though their gifter made a genuine effort to fulfill their requests. The ONLY things we absolutely insist on is that if someone listed something as a squick it appears NO WHERE, not even off-screen or alluded to, in their gift.
Everyone has to write in their first fest at some point. You are more than welcome to sign-up for the exchange to be your first one. Or, if you are nervous, snape_potter also runs several prompt based fests. Those are just all gifts to the community from a prompt of your choosing. So you don't have to worry that your gift isn't acceptable if that would worry you. We all do our best, but we're all here to have fun. I hope your first fest will be an enjoyable and as stress-free experience as possible.