Yes, you want to stay in the seat when you are climbing and you also want to have a light grip on the bars. This is a good rule for most (but not all) inclines because you'll be more efficient and gripping tight will cause pain later on.
i don't disagree with this. this is true for hills less than 3-4% AND/OR if you're spinning up with a low enough gear. on a fixie, though, on steeper hills you're going to have to get off the saddle and mash (or eat your ego and walk). in that case, leverage counts, that's why mtbs have wider bars.