Fsck returns your filesystem to a consistent state. This is not necessarily the filesystem's “latest” state, because that state might have been lost in the crash. In fact, if there were half-written files at the time of the crash, then the filesystem was not left in a consistent state, and that is precisely what fsck is designed to repair. In other words, after running fsck, your filesystem is as up-to-date as it can get.
If your application requires feedback as to what is stored on the disk in case of a crash, you'll need to do more work than just writing to a file. You need to call sync, or better fsync, after a write operation to ensure that that particular write has been committed to the disk (but if you end up doing this a lot, your performance will drop down, and you'll want to switch to a database engine). You'll need a journaled filesystem configured for maximum crash survival (as opposed to maximum speed).
The property that an operation (such as a disk write) that has been performed cannot be undone (even in the event of a system crash) is called durability. It's one of the four fundamental properties of databases (ACID). If you need that property, read up on transactions.
Although filesystems are a kind of database, they're usually not designed to do well with respect to ACID properties: they have more emphasis on flexibility. You'll get better durability from a dedicated database engine. Then consider what happens in case your disk, and not your system crashes: for high durability, you also need replication.
