Secret key Questions regarding expiration and backing up

[gpg at noffin.com]

Hi there - pretty new with GPG, but have been getting going with it
without much issue. I'm just curious about a few best practices and so on.

1) Should you set an expiration on your secret key? Or do most people just
secure it appropriately (with no expiration)?

2) If you do have the secret key expire, and I have a backup of it (file
format) - And for some reason I forget to extend it before expiration -
can I still extend it?

I did a few tests exporting a secret key before and after extending the
expiration date - and obviously the file contents changed. I just want to
be sure that I have a good backup of it, however follow best practices.

Thank you.