I think very. I've never designed anything based on arbitrary breakpoints (md, lg, xl and whatever numbers they might translate to) because that forces me to make design choices around those constraints.
The only way to sanely add breakpoints in my opinion is to gradually reduce the width of your page and search for things that start looking off. Are your paragraphs starting to look a little cramped? Add a breakpoint at that width, maybe remove the sidebar, maybe lower paddings and margins to allow it to stay a little longer, maybe manually reduce font size or switch your column layout to rows if applicable. Keep doing that until you get to ~350px width and everything looks fine. You decide what needs to change when it makes sense rather than being told that something needs to change at some specific breakpoint.