Also worth noting - the company may not mind you doing that work today but without anything in writing the company may come after you in the future. This is particularly relevant when you're on salary and work for a company that may be acquired or experience significant board turnover. I've had several employers who were very pro- side project and pro-OSS explicitly state that they'll approve anything that doesn't compete with the core business, but get it in writing for my own protection in ideal future of post-acquisition.