A mistake can occur while manually calculating your funds on each addresses. The best is to make use of your master public key (so far it is hierarchical deterministic wallet) to track the whole funds on the wallet with ease. But, why checking for this when the wallet already estimated your balance for you perfectly? You do not need to doubt the estimated funds on your electrum wallet balance.
It's more of a being sure everything is fine outside the offline wall thing. Just to be sure I'm still the owner of those funds 
So the only way to be in peace that everything is fine with my cold wallet is to check with my master public key?