If the Yasuo is at a point where he can clear an entire wave with a single Q, that's pretty late in the game. Doubling the value of a single minion at that point is not actually worth that much, compared to the things your team SHOULD be doing then. Clearing as fast as possible to either siege a tower or rotate to another objective is going to be worth more at that point. Also, if you upgraded to Face of the Mountain, it is more important for the shield, and the heal then scales with your health. Look for opportunities to kill a minion while you are peeling or something, because an autoattack from the support will generally do more if it heals someone like that than if it takes 40 health off of the enemy fighter who's diving your back line.
Now, that said, if your team is significantly behind and trying to make a comeback, they absolutely should be letting you use as many charges as you can. Of course, they should also be letting the waves freeze as much as possible to make the gold they can get from a lane last. If they instantly waveclear, the wave pushes into unsafe territory very quickly.
Much earlier, your ADC should be letting you get last hits with it. Hell, if I'm playing ADC, I usually ping a minion for them to take. Relic Shield has a cooldown of 1 minute, and the respawn time of cannons that early is actually 1:30. You should upgrade to Targon's Brace before the 20 minute mark, when the siege minion respawn is 1 minute, and your cooldown becomes 30 seconds, so until 35 minutes in, the siege minion respawn will not be in sync with your cooldown. The cooldown, however, also means that if you try to use it both on cooldown AND for a siege minion, you'll run out of charges.
The first wave of minions spawns at 1:30, and the first Siege Minion spawns at 2:30, so you can safely use a charge on the very first wave and the cooldown will max you out for the first Siege minion. After that, the next Siege minion spawns at 4:00, but your cooldown finished at 3:30. If you used it on a wave as soon as it was up, you'll use your second charge on the cannon and be empty until the wave that spawns at 4:30 reaches the lane. I would personally consider this a poor choice if neither of those two charges provide the heal, because between 4:00 and 4:30 is prime ganking time. However, by that point, the heal should be useful, so most of the mathematical preparation is merely theoretical here.
Mostly, it is important to simply keep track of the siege minion waves. Not only because that is the best use of the charges, but because that is also the best time to ward for safety and put some damage on the tower. Making sure your ward won't time out until AFTER a siege minion wave is good, for example. When you return to a lane, with both you and the ADC at full health, when the heal won't be useful for a minute or so, just look up at the clock as you walk back, do a quick calculation for the next siege wave. If it is the next wave, you are probably better off saving the charge. If it is not, then you should use a charge on a melee minion.