Say if you serving a white wine that didn't cost that much and is also awful. Your guest gives you a bottle of Pouilly-Fuisse and it's not gift wrapped. Do you say, "This is nice, let's have this." Or do you say, "Thanks, I can't wait to try this. Can I get you a glass of wine?" If it's that cheap wine you've been drinking, you're not being very gracious. When you know that your wine isn't as good as the wine someone has brought to your house, offer to open it. Let your guest tell you that's not necessary. If it isn't chilled, you can offer the wine that's already opened while the bottle chills.
If a guest brings you a really dreadful wine (probably a bottle someone brought to them) you still should mind your manners. Tell them you'd be glad to open theirs, but you have a wonderful white Bordeaux or great red they may like. We bet they'll be happy to drink your wine as opposed to theirs.
Champagne is another story. It can get expensive. Maybe the bottle that someone brought is a rare treat for you and you don't want to waste it on people who really aren't champagne drinkers. If it's in the box, keep it in the box. Or if you're in a small group and you know everyone's taste level, open it. But what if you're serving a sparkling wine and someone brings a beautiful champagne? This happened to me, Yvonne, last New Year's Day. I was serving Prosecco. A couple came with a bottle of Veuve Cliquot in a gift box. I was ready to open it. "No, don't. You don't have to, it's not even chilled." I thanked them profusely and continued to serve Prosecco. Had they brought it in chilled and not in the box, I would've insisted on opening it.
Then there's the serial bottle opener host. He or she has amassed a clumsy selection of wine from previous guests. They will go through their 'cellar' thinking nothing of serving something Italian, something French, something Italian and nothing for the ensuing headache. If you care about your well-being, this is the night to cut down on your drinking. You don't have to take the wine tour of the world, unless there's some place for you to spit it out.
A gift of wine is like any other gift. Most likely, you're in the company of friends and you should know what they like. If you can't afford a nice bottle of wine, don't go to the plonk section of the wine shop unless it's for yourself.
And if you're the host, don't go to the plonk section of the wine shop unless it's for yourself.