Since we don't have any canon examples, one way or the other, it's pretty much left up to our imagination. I think that it would most likely depend on two factors. How they died, and if their bodies were even recoverable.
Buffy died in Season 5, and was given a more or less normal, if secret, funeral, but it must be remembered, she died under very specific circumstances, and her death was witnessed by her friends. All Slayers die in battle, and not all die in front of witnesses, so the exact cause of their death might be in question. Would there be a possibility that she had been turned by a vampire, or otherwise infected by some other sort of demon, resulting in some sort of evil resurrection? In those cases, my guess is that cremation would be the preferred method of body disposal, with or without any sort of ceremony, and it wouldn't necessarily be a matter of love and respect, but rather pure practicality.
Also, there's the distinct chance that the Slayer in question might have actually been eaten by the demon or demons that killed her. In thhat case, obviously, no burial would be possible.