It doesn't matter when you have your bass high enough. My bass position doesn't change a bit when I change from sitting down to standing up. I tend to sit down most of the time when practicing, but I stand up when I have learned a song and now have to internalise it and "prepare" it for live playing.
For those who practice seated, I am curious whether you put your bass across your right lap? Or do you maintain the same position standing, meaning putting the bass between the legs? Putting the bass at my right right lap gives me a more relaxed position, but makes the angle unnatural for some of the positions. Putting the bass between my legs (same position standing) requires me to sit up straight! I think even Jaco put the bass across the right lap on the Modern Electric Bass IIRC.
Most of time I practice standing, but at the rare ocassions when I sit I play in classic guitar position (body on my left lap, left foot slightly elevated), which is the most comfortable sitting position I know.
I alternate positions when sitting down. Sometimes I find my left lap more comfortable when playing higher on the neck.
I sit when I'm trying to work out a new song or a complicated bass line, but I stand when I'm rehearsing for a show so as to get a feel for playing the song in show mode. I find that it doesn't matter how many times you play a song sitting down, when you come to play it at a show all your other stimuli are so different it can affect your playing, as nothing feels familiar since you'd practised in the comfort of a chair. If you're in a strange environment with lots of background noise it helps to be familiar with your bass in the actual position you're going to be playing it in at a show. Make sense?
Like JMX, I practice seated (even the upright bass...I use a stool). It's one benefit of wearing your bass higher, eh? I'll even sit on the gig if other band members are sitting (which is common at jazz and folk gigs). I have real problems trying to play standing if the bass is slung too low anyway. I really don't know how you guys do it.
I'll tell you a trick. Set your strap to make your bass the exact same height when sitting or standing. It doesn't look as cool, but your playing won't suffer when you stand up, and that's way better than showing off your sexy muscles (most of the time). Oh, and when sitting I either have the bottom curve part of the bass on my right thigh, or I have the bass rotated counter-clockwise so the cable is between my legs (I call that classical guitar position)
