You mean on different sources, right?
Yep, I got my LTC, doge, AKC, and XPM wallets backed up. But I just wanted to make sure that unlike disk or data backup, a wallet backup will pretty much "update" itself (referring to the transactions/balance).
Yes and no.
If you just have one main address that you use for receiving your mining proceeds, with the occasional transfer out, you should be ok.
However, most transactions generated by the default client also generate an extra address. (Assuming you are not sending exact totals of previous inputs.) This is known as a "change" address.
So let's say you have 35225 DOGE, which you received all in one withdrawal from your mining pool, and your wallet has no other transactions. You want to send 30000 DOGE to somebody. Your wallet client will send that 30000 DOGE to somebody else, and send the remaining 5225 DOGE to a new address. This address actually belongs to you, and the private key is stored in your wallet.
The wallet client will automatically create 100 addresses to start with. This is plenty for most people. Once an address is used to create change, a new address is created, so you always have 100 available and ready to go at all times. The private keys for all 100 of these addresses (plus any previously used addresses) are all stored in your wallet.dat. However, they are pretty much hidden from the end user.
So, if you have an old backup of your wallet, that has since been used for more than 100 outgoing transactions, chances are that your current wallet has some new addresses in it, beyond the first 100 stored in the original wallet.dat. They may not hold much, if anything; most people use one main receiving address, and the extra "change" addresses only hold dust. But, it could add up.
So yes, you need to make backups of your wallet(s) on a fairly regular basis. The original Bitcoin-Qt client does this, as do all clients based on the Bitcoin-Qt code (Doge, Litecoin, etc.). For oddballs like NXT, I don't know.
https://en.bitcoin.it/wiki/Change
http://www.reddit.com/r/Bitcoin/comments/1c9xr7/
http://bitcoin.stackexchange.com/qu...ansaction-outputs-when-sending-to-one-address
http://bitcoin.stackexchange.com/questions/4239/what-exactly-happens-when-a-new-wallet-is-created