I'm a spare tank freak, too. Have 4 of em' right now, but would be more comfortable with 5 or 6. Have 2 LPG smokers now, so if (when) I get them both rolling along with a grill or 2, well, thinking about it makes me nervous. Imagine 25 to 30 people (1/3 or more of them adults) having to wait a bit longer for the meal that they have patiently, yet anxiously been awaiting. And I'm off (or most likely sending someone) to the local propane distributor for a refill. Not me, don't wanna go there!
Anyway you want to do it, the extra insurance will cost you. All my extra tanks were between $24 and $28 at wally-world or sammy's-clubber. So, figure a bit over a hundred for 4 and all I do for regular cooks is make sure that at least 2 tanks are good to go. When checking my equipment between cooks, if I've got 2 empty I get 'em filled that day. I also keep a good store in the winter, as I have a propane forced air space heater in case all h*#l breaks loose. We frequently lose power during storms here. I now have 2 generators ready too. For me, just having the extra LPG on hand is just good peace of mind.
The bottle scale idea is good for situations other than mine. I actually owned a Thermos grill (purchased in 1994) that had a bottle weight gauge built into the grill and I thought it was a joke 'til I used it. They do work fairly well. The hand heft test I use is to slosh the propane bottle to feel how much fluid motion can be detected. The weight is pretty hard to feel with a near empty bottle, as a 20# bottle (5 gallons total) with 4 lb/gallon propane weighs in at approx 47-48# when filled to 80% (it's mostly cylinder weight). The slosh test can reveal down to a quart or 2 for me if I really need (or want) to stretch it, but I pay by the gallon, not by the bottle for refills.