I do log out of financial sites, as others have mentioned, but any decent financial sites should use cookies that expire when you close your browser and also after a short time of inactivity, so even if you forget, you are generally safe.
For the record, I don’t get any spam. I have never had any virus, spyware, malware, or trojan horse on any computer I owned except once when I used a program written by a hacker on whom some other hackers had played a practical joke by inserting a virus in his program. That was in the nineties. I honestly don’t know where people get all these problems unless they are signing up for offers and giving out their email, or downloading questionable files, or have their email addresses posted somewhere publicly.