I might find that for instance a word comes out with a few italic letters amongst the roman ones, and ctrl-I won’t clear them
Please elaborate your description (see these guidelines). We’re not behind you to see your screen and what you do. Are you typing your formula with the Math component (Insert>Object>Formula)? Manually?
Provide a screenshot through editing your question. Don’t use an answer which is reserved for solutions.
If you create italic text as direct formatting with Ctrl I the best way to return to normal text is to press Ctrl M (remove manual formatting).