I had the same concern when we bought ours from NordicTrak -- it was rated for 275 and I was about 290. My husband (an engineer type) also said that these things are built for certain tolerances over the limit and said I should use it. I did, but was nervous doing so. And then we did have a part break early on (apparently, when something goes, it often goes early...). It was still under warranty and so was fixed,
no questions asked. I will say that since I've gone below the weight limit it squeaks a lot less than it used to.
I would also recommend you call the company and see what they say. To cover their

though, they may just give you the party line. You could also use that time to ask what activities are hardest on it -- does speed matter? Incline? I think my husband would say no, it's all the same, but he's not here...
If you do use it and something does happen, they may also fix it without asking questions.
Edited to add: I did mention it to my husband, and he was much more hesitant about the difference from 290 to 250 than he was from 290 to 275... FWIW...