For the most part I like tip up. I can orient my hand better on the initial grasp of the knife which makes deployment faster for me. There are a select few that I like tip down like the BM Barrage.
I generally prefer Tip Up carry for knives carried on or below the waist, and Tip Down carry for knives carried above the waist. Although a specific placement with a specific knife/deployment design might dictate whether it will be TD or TU, regardless of preference.
Tip up since it flows with my training. It's also rather easy to just flip the sheath and knife to tip down if you want to do a low draw slash, at least easier than it is to flip a tip down knife up if you're deploying it against the wrists or something.