If I backup my wallet.dat file, is there a certain number of transactions that I should re-backup the wallet, or is it perfectly safe to leave it forever?
That depends on what wallet program you are using, how many new addresses you generate in the wallet, and how many of the transactions are ones you sent (rather than received).
If you are asking about Bitcoin-Qt, by default settings it stores the next 100 addresses that you will use. Every time you send a transaction, the wallet sends the change from the transaction to a new address that it pulls from that pool of 100 addresses (without telling you that it is doing this). Everytime you choose to generate a new address, it pulls the address from that pool of 100 addresses as well. The wallet does not give you any indication of how many addresses you've used since your last backup.
Therefore to maintain a margin of error, I'd suggest always keeping at least your 3 most recent backups. I'd suggest creating a new backup whenever the sum of addresses generated and transactions sent exceeds 25.
