There's been a number of changes since the version you've got installed was first issued. iTunes is now at version 6.0.4 (3)
Recent changes have addresses iTunes crashing (on some computers) when opened, and the main change to the last version was some bug fix for connecting iTunes to remote speakers and minor bug fixes (Apple never say what these are).
The main downside to not updating is music purchased from iTunes. Apple are constantly updating the digital rights management system that protects purchased music so if your version of iTunes is not current, then it may cause problems with the playback of that music or transferring it to your iPod. And contrary to what's been said earlier, you do not need the current version of iTunes installed in order to update the iPod's software. Neither is updating going to affect the quality of your music as the codecs have not changed.
If none of this matters to you, stick with what you've got.
I don't want to get trapped into some marketing ploy requiring constant updates and possible future subscriptions.
I'm not sure what you mean by this. The use of iTunes to import your own music and then transfer it to your iPod has never required a subscription and never will.
For what it's worth, I've always kept my iTunes current and never experienced an issue after updating.