We did ours a bit later than we should have but really once they can sit up they could pull themselves up so better safe than sorry. OT but I was in the worst habit from when she was little of leaving the side down - when she couldn't sit up it didn't matter but then she started sitting and a few times I went in after a ap and nearly had a heart attack seeing her sitting there and the side down.
Needless to say now that she is standing I've never once forgotten!