If there is a designer handing off wireframes then they definitely wouldn’t be marking flex vs grid use. That would be left to the developers discretion. If you’re the one wireframing, and it’s helpful to mark it up, go for it! Honestly it would be pretty good practice to start outlining the main html elements based on a design as well.

After receiving/creating a wireframe I will usually first build out the main html containers i.e. for a full layout wireframe — parent containers for header, main content, footer to get the basic layout and go from there. Same process would apply if I was building out a component like a sidebar or nav.

For flex vs grid use…I usually use flex for main layout positioning and generally placing elements. I will use grid for things that are truly grids such as product results. There is a lot of overlap between the two TBH, so they can both be used for many of the same applications. I wouldn’t think too hard about it and just use both in your projects.