Ballymaloe and Jamie Oliver both advocate mashing most of the strawberries, leaving only a few whole ones for interest's sake. Johnny and Nick leave the smaller berries whole, and chop the larger ones up, and Nigella doesn't bother to do anything to them, although she allows you could cut them up "if you like". I find too many large berries make the finished jam more like a compote, but scooping out the occasional baby is a real treat sadly lacking in most commercially-produced examples.