Keychain cleanup before sync
No on a technical level. If you know how to clean up your keychain, you can do it as well after you enable sync as before. 

Yes on a philosophical level. You might have items you don't want in the cloud where the chance of error, surveillance or disclosure are higher and your ability to permanently erase the data is lower. 

So, I would only clean an item you don't want on Apple's servers directly.