I like when people who know me and the context well enough to be 99.9% sure they have the correct word say it, because as soon as they do something happens in my brain and suddenly I can say it and continue my sentence. It's a relief to be honest, but I have had specific conversations with most of these people around doing this and they know not to keep guessing if they get it wrong and things like that. That said, it's generally best to err on the side of caution and assume people don't want to be interrupted, as this can feel like you're hurrying them along. So answer your question, it is more polite not to say the word. But if this is a regular customer and you feel comfortable doing so, you could ask if he would like you to say it in the future. Or if he has a regular drink, learn what it is so you can offer him his usual without him needing to say it. If offering "the usual" isn't an option here, another option in your context might be to indicate your guess in some way other than speaking. I'm picturing you serving at a cafe, so if there's a way you can point to the sweet tea or something that could be the way to go. For example if someone wanted a vanilla latte and is stuck on the "V", I'm picturing you pointing to the vanilla syrup and then they could nod and move on to "latte" for example; similarly if someone is trying to say "earl grey tea" you could tap/lift the tin of earl grey tea bags. All while trying to keep a friendly expression on your face as much as you can so the person doesn't feel hurried.