I greatly doubt it will mess up respawning as the ash piles are a seperate entity than a corpse or the actual npc. the only issue with non termninating ash piles is that they can potentially bloat your file size which at around 20 mb will make the game unplayable but keep in most players will never reach such a file size with the implemented features in 1.4. this file bloating has been around since just about forever and it is incredibly disheartening to know it still exists several game iterations later.
this mod is just about mandatory for anything investing more than 200 hours into a single char.
http://skyrim.nexusmods.com/downloads/file.php?id=4528