I think all pickers will agree they are able to play better in a sitting position. Standing forces you to deal with your balance, the weight of your instrument, and an awkward playing angle among other things. That being said, there are times when standing is the only option.
If you are playing with a band, standing is pretty much unavoidable, as presentation is key when trying to impress an audience. Standing displays well, as it is necessary for the movement showgoers love to watch, and the presence you need to project. Choreography can be especially mesmerizing, when playing the old fashioned way, into a single mike, with break players taking turns moving up to it, while it's former occupants move away, and then back. You can't do that sitting down :)
Many jams I've been to, especially circles which pop up at festivals, either don't offer sitting accommodations, or exclude you if you're not prepared to play in a standing position, as you won't be able to work yourself into that circle otherwise.
It's better to spend a little time, effort, and thought to figure out what works best for you. We all want to be ready and able to play our best, regardless of our degree of ability, and mastering this little stretch of guitar playing territory is another notch in your belt.
Getting It Done
Finding the guitar angle, height relative to your body, and arm angles relative to the instrument which gives you the most comfortable picking and fingering positions is the challenge you have to meet when standing while playing. Experimentation is the only way to determine what works for you.
It's not rocket science. Move the neck up and down across your body, and play in different positions until it feels right (if you play a lot in the upper register, it's usually better to point it more upward than if you don't).
Adjusting the strap is the other factor in choosing your position - it will determine where the instrument sits on your body. Keep mental track (or better yet take pictures - use a web cam or a regular camera using the self-timer if you don't have help) of the positions which feel best.
Find the combination of the two factors which fit you best, and play a number of different tunes. You may find slight adjustments for individual songs are advantageous.
I realize the guidelines offered are somewhat vague. That's by design. I've read a few articles on this subject, and believe the information they offer is too scientific and specific. Positioning is such an individual task that doing anything but pointing a player in the right direction is overkill in my opinion, especially where flatpicking is the category.
Please don't sell yourself short. Experimentation should be a never ending enterprise. The day you think you've got everything you need to know down pat, is the day you've given up trying to improve. What works today will still work tomorrow, but perhaps not as well as something new you might try.