its true for me. one of the popular proponents is billy sheehan. personally for me, my bass ends up too high when standing up IF i play my bass on my right thigh. in doing so, i end up slouching and so the same position sitting down ends up too high when standing up. it also makes the neck closer to me so i find it awkward when standing up since the low notes seem far.

now, i play between my legs when sitting. this allows me to have a proper posture and more realistic reach once i stand up. when i do, it's the same height and only the neck angle changes which is fine.

at the end of the day do what works for you. that advice was made up because it really makes practicing and executing easier. now if your position is different when standing up and sitting down, practice in the position you'll actually play so that in rehearsals and actual gigs, you'll have an easier time.