You have to be online to continually receive new transactions. If you generate a block with missing transactions, it will be rejected by the network.
I assumed you want to be online to get notification of what the latest successful block is so you can chain to it. The block you want to chain to changes at any time, on average once every 10 minutes.
I thought you can choose to not include some transactions, hence the need for fees. Am i wrong here?
In this case Cryptoman is wrong. You don't have to include ANY transactions, but if you are not working on the latest block it will not be accepted.
