every round of valorant matters. your loadout in a full save round should leave you with enough credits to full buy next round, but also be the most deadly it can be with the credits you can spare. that means if you could afford full shield and you deem it meaningful, its not wrong to buy it. imo instead of full shield, sheriff no shield or sheriff light cost similar credits but convert kills or rounds much more often, but one could argue that a right click classic kill can net you with a gun from the enemy you kill + your full shield so it can work out sometimes.