I have an electrum wallet and have exported the Master Public Key. What level of security should I use for the MPK? Should I keep it offline? Can I keep it in plaintext in an online computer? Back it up somewhere ( gmail / dropbox )?
xpubkey (your master public key) is not something you need to export.
What you need to store is your seed value for the Electrum wallet. With this it can regenerate you the
xpubkey is not really useful unless you are providing it to someone who intends to make multiple payments to you. If this is the case then make sure only that individual or business will be sending you funds to the address set as once the
xpubkey is shared it allows them to see all addresses down that chain.