It depends upon the situation, especially since starting off with BO isn't good if you're going to hang around to fight, because of power levels (but I do love adding the de-cloak bonus to BO). Actually, for the one from my sig, I was rushing into the battle because the match was about to end, so my inertia carried me to about 3 km away from my target. Not even sure if my weapon power was at max, and I think I only had a few buffs up because the rest were on cool down. Furthermore, I remember hitting a 37K crit last night in a private challenge, and I was probably something like 0.2 km away from my target. So, closer, like with cannons, is probably better. I remember a lot of crits around 18K, so if they were from 10km out that would be consistent, but my memory could be foggy - so many details to remember from so many battles. Only the ones at 30K and above really stand out at this point.