you never need to open the radiator cap unless the overflow tank is showing abnormal level, meaning its either overflowing or is completely empty - thats when you need to check the radiator cap.
This information is also in the book thats in the glovebox of the dashboard.
if you remove the cap while the car is tilted (parked on slope) the level will be incorrect. And it will take a few drivecycles to burp out any ingested air. So just leave it alone and watch the overflow tank level. Do not use tap water to top off because "buss yehi thaa uss waqt"
hard tap water minerals will destroy japanese specification coolants like genuine suzuki green, toyota and honda. They literally form crystals due to that.