I tend to be wary of embedded CBs showing in the forecasts - it's not so bad if you can see them and navigate around them (for which I do the old 60 degree 2 minute dog leg).
So no hard and fast rules as it depends what kind of front is causing them too.
Isol, few, or CBs around a particular front all could indicate flying can be done.
Having said that, a building Cu (not CB) on one of my first solo navs once gave me a couple of severe jolts that made me give the big 'uns a bit more respect.