This is not so simple. Do you expect that someone can come to your home and take your machine and read your data? If yes, you probably should use TAILS, not Qubes. Do you expect that someone can infect your BIOS while you leave your computer unattended? Then, you should use Heads or AEM.
Do you think that state actors are trying to get you? Then, you have almost no chance to hide. Enter your passwords under a blanket, live in hotels for no longer than a day, and it will not help you.
Now, realistically, for 99% people default Qubes install should be reasonably secure. You can use minimal templates to further reduce your attack surface and do other things mentioned above. The best approach with Qubes is to compartmentalize your digital life as much as possible, to defend from online threats. There is no end to hardening, but it’s a good idea to stop somewhere.