Good grammar matters because there's always someone keen to put you right if you get it wrong - even in the least likely situations.
My husband Adrian was doing his final pre-marathon training run over the South Downs last week. After 20 miles, he gasped that he wished there were less gates to go through.
One of his fellow runners shook his head sorrowfully. "Fewer gates, Adrian, you mean fewer!"
He was, of course, right.. The basic rule is that you use "less" if it's something that can't be counted. If you don't want so much mash, for instance, you'd ask for less mash. But if (more likely) you'd rather go easy on the peas, which can be counted, you'd ask for fewer peas.
I bet on marathon day he'll be wishing there were fewer miles...