5% is still significant, but in my experience it may be more like 10-20% of the sites I visit where it matters (eg. financial institutions, government, internet service and cellular providers for account info, etc.I'm pretty die-hard with Safari. If I had to estimate, I'd say maybe 5% or less of websites I go to work badly or break with Safari. I guess everyone's personal mix would be different, but the vast majority of what I do works just fine. Still, yeah, you gotta have a backup browser around.
What's worse is when it seems to work but actually doesn't, in a way that hides that it is not working properly. For example, for my pension plan website, the web page looked normal, but it was actually missing a few options on the page. I ended up calling up the telephone support and they walked me through it. When I mentioned I couldn't see some of the options on the page that they thought should be there, finally I switched to Chrome and everything was there.