Need to distinguish between coin/supply instants, and time instants.
Coin/Supply instants - definitely not a waste to hold a few of these in your inventory. First, you'll definitely run out at some point, especially in later chapters; why not wait until then? Second, if the "gain X supplies or X gold" quests come back, bam! that instant just became a completed quest.
Time Instants - More arguable. But its opportunity cost. E.g., I have a 14-hour instant. I could immediately upgrade a Fairy House to an Orc House. Or I could let it run its course overnight tonight as planned. I get a house 12 hours sooner. But what do I do with that extra house? Probably nothing. I'm probably just building another house upgrade.
There are definitely times where rushing a production saved me many more hours. For instance, I had a 14-hour build needed for "upgrade 2 buildings" quest. I completed that quest with an instant, then ripped through 5ish more easy quests basically immediately, which allowed me enough Acorns to open 2 more large chests. I got a nice daily building in those chests that I'm otherwise missing if I waited for the full build.
So in all of my rambling, I'd say: save a few coin/supply instants until needed. Save some time instants, but you probably don't need to sit on 7 14-hour reducers. Just spend all but 1 or 2, and save a couple for strategic benefit.