td00Not necessarily. When I first got my my Kaweco Liliput BB, I left it nib down all the time because I'm pretty sure I got a baby's bottom nib and it had a shit ton of starting issues. Never had ink creep out. I've since bought some micro-mesh pads and sort of fixed the baby's bottom issue (when held at a specific angle) but I still leave it nib down since it's a BB and very hungry for ink.
That being said, it's been stated below that this doesn't exactly fit most fountain pens; so pertaining to ballpoints and rollerballs, I keep those in my pockets facing down as well and never had any ink leak issues.
TanvirK175Fountain Pens should always be stored nib upright so the ink moves down the tip (the tines) so it remains smooth when writing and does not clog. I hope this helps everyone!
Happy Fathers Day, Dads!!!