Maybe after a gap of a few years when it's all really cooled off, but for the most part I doubt it. There's almost always too much emotional baggage tied up in it for you to have the regular level of nonchalance you need to be fuck buddies. My guess is that you're wondering now because you liked having sex with your ex and now that there looks like there might be an opportunity you're just trying to find a way to rationalise it to make it seem like a good idea.

Which isn't to say 'don't do it'. Worst thing that can happen is it backfires, it's a bit painful and you end up learning something from it.