I'm pretty sensitive to the taste of lactic in the finished beer, so I limit my total input to under 4 ml or so.
I'm more sensitive to the aroma, which is why I changed to phosphoric. My LHBS gets the 75% version, so amounts are pretty close to the same (I use Bru'nwater to calculate amounts), but no real impact on aroma/flavor.
I've gone to phosphoric, too, but I'm using the 10%. I've heard that the 75% is strong enough to be borderline dangerous. I don't ever really need to use that much, so the weaker (safer?) stuff suffices.