If we consider standard five or six training zones in TraningPeaks, those will normally have a spread of several Watts or BPM. Does it make a difference if I train high or low within that zone?.....
.......I understand that of course in reality those boarders are fluid and the 5/6 Zones are just to simplify things, but 170 and 230 Watts is quite a difference. Or should I aim to be exactly in the middle of my zone?.....
I think the question doesn't really make sense given these two conflicting ideas.
What I mean is, the zones are just approximate ranges selected from the continuous power/HR spectrum from stopped to flat out. Which you appear to agree. So yes, it matters where within the zone you are working, because zones are not a switch where you're doing the right training or not doing the right training. Higher in a zone is just the same as higher intensity anywhere on the intensity spectrum. The zones are not "real", they're just a convention to facilitate communication.
So, what I'm getting at is this: The real question is not whether it matters. The question is what the session is supposed to accomplish and/or what the person who designed the training session intended that you do (whether that's yourself, a coach, or some other training plan).
I totally agree with this. If you are looking to do a completely aerobic workout, first you would probably use HR (BPM) instead of power. HR will tell you how hard your body is actually working, while power tells you the output your body is producing.
If you are looking for the exact HR where your body is still completely aerobic, you will probably need a metabolic efficiency test to find your anaerobic threshold and train below that number. Or you can estimate by subtracting 30bpm from your lactate threshold. The goal would be to train your body in a state where you are burning mostly fat for fuel instead of using sugars. This will increase your aerobic efficiency.
In short, you might want to aim for the low half of your Zone 2, because training anywhere below the threshold (1 bpm or 20 bpm) will give you aerobic benefits.