Barty suggested that he and Bella burn the bodies (interesting, given that he must have known that Dolohov wouldn't be too happy about it). Bella said she didn't care. They did whatever they did off journal, but Dolohov was told the bodies were burned.

I believe Bella and Rod decided to hold on to them (ew, by which I mean put them in a secure location) for a couple of reasons:

1) leverage for later, if needed (as employed by Rod in his plea bargain)
2) Both of them wondering if Voldemort's remains might have some mysterious power or value attached to them, given what an awesomely dark wizard he was. Rod was an experimenter, remember.