I Open Source whatever I'm allowed to open source, when I do contract work for companies, to address this very question for my future self. I usually include a clause in the agreement that the code will be released under an open source license (they can pick, or I can pick Apache license or GPLv3 with copyright held by me and them receiving an unrestricted license, whichever is appropriate to the project), unless explicitly requested otherwise.

I've found it is rarely an issue that even raises eyebrows these days; unless the company is a software company selling software as their primary business, they don't care what the license is as long as it solves their problems and won't give them legal hassle down the road. Most companies just need solutions running on their servers; they don't care how the solutions get there.

If something is clearly work-for-hire, and isn't Open Source, I wouldn't re-use it without explicit written permission. But, I can't remember the last time I did work that met that description.