I started a thread on the Rogue subforum as reference for the non-TC crowd, but when I found a small bug, I decided it was time to fire up a dedicated thread here too. I crunched some numbers and ran some tests, and the +20% Bleed Damage affix is giving +20% Base Damage. Other bonuses listed in that fashion are multiplicative bonuses unless explicitly stated as Base Damage bonuses.
Another small quirk is worth noting. Bleed damage is typically a damage over time effect. In the case of all two powers this bonus applies to -- Vaulting Uppercut and Crimson Sweep -- the powers are designed to deal immediate damage as well as a DoT. In both cases, the bonus is treating both the initial impact and the DoT as "Bleed" damage and giving them the +20% Base Damage bonus. That's probably just tags being tags and expected behavior, but I thought I'd mention it.
I tested it with a couple items (Lady Deathstrike Medallion and Maht's Bladed Arrowhead) and neither benefited from the bonus. I believe that's intended as item procs almost never respect character bonuses, but I wanted to put it out there.
I'm not sure how many builds such a bonus is really going to facilitate. It doesn't really open up any interesting power interactions; it's just a passive damage bonus. What's more, it only affects two powers, greatly limiting its appeal as a passive damage buff. You can't really build a "theme" out of that. Crimson Sweep is already very popular, due to being the best Vulnerability layer available, and needs little incentive. It may give reason to include Vaulting Uppercut, which would be a very respective damage layer if it were to get an extra 20% damage multiplier thrown on. If it stayed an additive bonus, not so much.
What might be more interesting is some kind of direct interaction with the Bleed status. Affixes like "Damage Rating vs. Bleeding" are already in the game. Spirit on Hit vs. Bleeding enemies or the Maht's Arrowhead affix of restoring Spirit when you deal damage with a bleed would be very appealing to players having trouble managing Spirit, since they could easily pair it with Vaulting Uppercut and not have to chase down a bunch of powers to make the combo work.
Or... y'know... There's always dat Living Ribbon toggle... Just sayin'.