The way I look at it is, if I'm keeping myself safe, that is by definition not obstructing. When we (and, I think, the law) speak of being "obstructionist", what we really mean is unreasonably so; being "in the way" without good reason. When there is a good reason, it's merely being part of traffic. Traffic has to wait for other traffic all the time; it's part of driving.

For example, when I'm taking the lane at a red light, I'm technically not obstructing because the traffic behind and in front of me is not going any faster than I am; we are all stopped. So I'm not technically "holding them up". (And, whether they realize it or not, if I weren't there on my bike, I'd be there in my car, taking up even more space!) If the cars behind me have to not accelerate quite as rapidly as they otherwise might when the light changes, it's for the perfectly good and legal reason that they have a vehicle in front of them (me) which needs to get through the intersection safely. Now if I intentionally moved slower than I could, I am obstructing, but if I'm going as fast as can be reasonably expected for my mode, I'm just another vehicle they have to take into account.