I would upgrade all of them just because.. I can!
Seriously though, it depends on what software you actually use. If you never compile anything there's not much use in upgrading gcc for example. And if you don't use X, well, you get the idea.
It's wise to read the changelog on the Slackware website, it will tell you if an upgrade fixes a security issue or broken Slackware package, or is just meant to include whatever new functionality the developers added. Be advised though, that usually developers will include bug- and security fixes in their releases so even when the Slack changelog doesn't explicitly specify that an update is a security fix, it might still be worth while.
Short story: do some research on what it is you're upgrading and what is different about the new version, that should be the base for your decisions, rather than the list of things slapt-get feels it should upgrade.