basically, with QT, you need to back up when you send 100 transactions, correct?
As I said, backups a bit more frequent would be safer, just in case your most recent backup is somehow unrecoverable.
You'll have to decide for yourself how much risk you're willing to accept, but I find that a backup every 25 transactions and storing the most recent 3 backups in 3 separate locations satisfies my risk tolerance.
Note, that if you are generating new receiving addresses in your wallet, you need to consider those as well as the transactions that you send.
So for instance, if (after backing up) you send 18 transactions, and you generate 7 new receiving addresses, then you've used a total of 25 of the addresses from the pool of 100 addresses that are stored in your most recent Bitcoin-Qt backup.
Does multibit work the same way?
I don't use Multi-Bit, so I'm not sure. Everything I say about it here is based on an educated guess and is not reliable, so you may want to wait for someone more knowledgeable to stop by and answer questions about MultiBit before making any decisions:
As far as I know, MultiBit does not use hidden "change" addresses like Bitcoin-Qt does. Therefore, there is no need to create a new backup when you've been sending transactions.
On the other hand, as far as I know MultiBit does not have a pool of pre-generated addresses for inclusion in a backup. Therefore you would want to create a new backup EVERY time you generate a new address BEFORE you have any bitcoins sent to that address.