It really just depends on your style.
Personally, I wouldn't cleanse it. I would continue to reinforce the necklace's purpose from time to time by either enchanting it again or "charging/feeding" it your personal way. But that's my approach.
Another approach could be to clean it from any junk it may have picked up, then repeat the enchantment (not because it went away, but because this step will reaffirm it).
It's entirely up to you. Just listen to your gut.