I think the decision about using Title Case against sentence case can be made using this rule of…
Ganesh Kulkarni
301

For most things, I think this rule of thumb can work. But things can get a little tricky when it’s not clear what’s a title and what’s not. For example, would you consider a tab name a title? Or how about a confirmation message that shows up for 5 seconds and says “Email sent”/”Email Sent”? Those are some of the cases where I feel like different people have different opinions. It’s always interesting to hear what people think though!