Most people (pretty much everyone, in fact) recommends that your picking hand should be as relaxed as is physically possible, but for me, I can pick faster and more accurately if I tense up a little.

Is this bad practice, or should I just carry on doing what's working best for me? I just don't see how you can be relaxed and play as efficiently.

That just means you haven't practiced on it enough.
I know what you mean by when you tense up and can play faster but really
once you practice playing relaxed everything becomes so much easier to play instead of having to get ready and tense up for it which will in fact tire you out faster.
I can tremolo pick faster if I tense my hand up, but normal rhythm playing sounds terrible if I'm not relaxed.
1978 Peavey T-40 -> Ampeg Micro-VR - > Ampeg SVT210AV + Ampeg SVT-15E
Well, you should be as relaxed as possible. If you're right at the edge of your technique you'll feel tense before you lock up, but trust me -

Tension just holds you back, and the longer you play with it and the tenser you get, the harder the speed roadblock you'll hit, the worse your accuracy, and the higher your chance of injury.

Just how it is.