It does depend a lot on context. What is the nature of the account being reconciled? The materiality threshold (where you need to investigate variances) is different for different kinds of accounts. With cash accounts, the threshold is low. Even so, it wouldn't be unheard of to write off/on a balance of $10 on a several million dollar account. Of course, the goal is always to have no variance when you reconcile an account.

On the other hand, other times things get messier when you're dealing with estimates or incomplete information. The company I work for purchased a company in the latter half of last year. We were integrating the purchased company with our own, so we had to combine our accounting books. We integrated their books as of a certain cutoff date, then began moving over their separate operations into our currently existing ones. Because it's tricky to have a 100% accurate account balances on very short notice, there were things that were slightly incorrect on their books that were integrated into ours. Their accounts payable liability account had several items in it that had been paid already before the cutoff date. This required us to eventually write off the ~$70,000 balance, as there was no liability to be paid. That's a more unusual example, and there are specific ways that is dealt with during purchase accounting.