This is tricky one - because I'd like to answer you that you should keep on the bpm you are at, without pushing speed at all. This will make progress a little slower - but once you do get there your playing is going to sound better than 99,9% of all shredders.
However since I know that most people get bored when practicing day after day with the same bpm - my answer would be to push the bpm speed just so much that you remain inspired. But if you overdo it... you might get separated from the world of tone forever