Originally Posted by
Airburst
Well, it's not particularly good for your knees, and it will put more force on the crank/spindle interface, but if there's not something wrong already, I doubt it'll do any harm, assuming you haven't got absolutely ridiculous amounts of leg strength.
For a given gear combination, standing gives more leverage and allows the knees to be straighter than if the rider were to stay seated, so I'd say it's better for the knees.
That said, if you have dodgy knees, it's best to shift down if you find yourself needing to stand to keep going forward.