It's not safe to use listtransactions this way.
I know I've been criticized for being reluctant about listtransactions. Let me explain my reluctance.
Transactions are dynamic. Past transactions can become unconfirmed, go away and come back, become invalid and disappear, or be replaced by a different double-spend. Their date can change, their order can change.
Programmers are naturally inclined to want to use listtransactions like this: feed me the new transactions since I last asked, and I'll keep my own tally or static record of them. This will seem to work in all regular use, but if you use the amounts for anything, it is highly exploitable:
1) How do you know if a past transaction becomes invalid and disappears?
2) When there's a block-chain reorg, it would be easy to double-count transactions when they get confirmed again.
3) A transaction can be replaced by a double-spend with a different txid. You would count both spends.
The model where you assume you only need to see new transactions because you've already seen previous transactions is not true. Old transactions can change at any time.
Any time you take an action based on payment amounts received, you always need to go back to bitcoin and ask for a current balance total (or use move or sendfrom), and be ready for the possibility that it can go down.
Now that we have the Accounts feature making it easier to do it the right way, we're better prepared to have listtransactions.