I'd just leave them sloped. I wouldn't strip the whole lot in one go, though as that could lead to some soil washing off in heavy rain. Maybe do a couple of 2m length on each side at a time, and build it up.
Once your plants are established, they will stop any risk of run off [monsoons excepted!]
So have the borders sloped? Or try and level them? I thought I'd have drainage problems either way. Great if not though!