I have an 'Electrum' wallet. I found some old public keys and imported them into it and then sent the bitcoins to an address in the main wallet. Now the transactions are confirmed and there are 0 bitcoins balance in the imported keys I presume it's safe to delete those keys from the wallet? Is there any circumstance in which I'd ever need them again? (Assuming of course nobody is going to pay anything to their addresses)
You can delete them with the second click options if you think so.
If you ever receive coins in that imported addresses you won't know if you don't have them imported in Electrum or in other client. Or if you've registered somewhere signing messages with it (like in #bitcoin-otc)
In case you want to keep them you will have to keep a backup of those keys separately by copying the wallet file as they won't become part of your Electrum seed.