It's really hard to get much better than 70/140 BPM now that it's the default BPM when FL Studio 11 starts up But seriously, 70/140 BPM is pretty ideal if one actually exists. But I'm always surprised when I time a tune and it turns out to be 60/120 or 75/150 or whatnot. I think ultimately it depends upon the tune. But true DubStep and not BroStep usually needs to be slow enough and not rushed. Some of the hybrid subgenres can take more liberties. A lot of true Reggae and some Dub Reggae is at 69 BPM which is almost a perfect 70/140 so it works perfect for the DubStep samples and influences.
Why even bother considering creating tracks with a fixed BPM in mind it feels uncreative to me
I concur. Though it would be more of a concern if you were planning on mixing dubstep. As annoying as it is to pigeonhole styles the majority of dubstep is at 140 =/- 5bpm. Some of the earlier stuff stems from garage and could be around the 135 mark. Alternatively Youngsta was playing everything at 142 for a while, when that dungeon sound was more popular.