It makes sense to do your crosses early if you can, but I can't seem to live with a cut off date.
At the end of season, I find myself admiring the plants that are still standing - whether by my choice or by being most resistant to the various tomato plagues. So even though my season is functionally over by October 10 - not enough hours of sun in my greenhouse by then - it's becoming a habit to do last minute crosses as late as mid september. Agonize over the slow progress or revel in every sign of growth
until finally either the branch or the fruit has to be brought indoors, and slowly finish ripening usually ending in a paper bag.
Some of the late crosses I've only gotten a few seeds, but what the hey, you only need one F1 plant to get lots of F2 seeds. I was saving seeds from late crosses this year, in November.