I have a suspicion that pitching rate makes no difference to the flavour of beer. Brulosophy did a pitch rate experiment with lager and found nothing. I think the theory that yeast produce different levels of esters at different cell counts is possibly wrong.
This is completely false.Just because of one Brulosophy experiment does not make it an absolute. Underpitching, for instance, can cause ecessive cell growth and can cause head retetntion problems due to excess fusel alcohols. Over-pitching can certainly cause autolysis which can easily be determined by a higher pH in the finished beer (or a raise in pH in the finished beer.) Pitching rate also will affect the pH in the finished beer.
While I agree with Denny about acetyl co-A, I still find that if I over pitch my Belgians or a Koelsch in particular it affects (lowers) the ester production, especially in the case of esters I like and look for in those beers.
I have found the MrMalty calculator to be very good at calculating fresh slurries of yeast. It pairs up well with my own cell counts and viability tests. So there is no reason not to use it.
If you have a slurry older than a couple weeks old and don't have access to a microscope/hemocetometer I recommend either making a starter from that slurry or just starting fresh. One of the worse things you can do is over pitch a beer on purpose because you fear the viability is low then bring in a bunch of dead cells.