is it better to force close apps or leave open

Yes, it is true. Open apps do not use any resources when they are not actively on your screen, unless they are specifically created to do so. Even then, iOS puts strict limitations on what they can do in the background. An open but dormant app can come back to life instantly when you open that app again, right where it left off. If you force close it, the app must start itself again before you can use it. That requires extra processing, which consumes power.

Leave them open. The only time it may be necessary to force close an app is when it is stuck or misbehaving.

edclange already explained this to you.

An open but dormant app can come back to life instantly when you open that app again, right where it left off. If you force close it, the app must start itself again before you can use it. That requires extra processing, which consumes power.

As you can imagine, extra processing requires extra power, which places more needless strain on the battery. The less strain that you can put on your battery, the cooler it will operate, recharge and the longer it will last.

It won't really "hurt" or "damage" the battery to force close apps and then restart....(except for requiring more battery power)....but it won't "help" either, if you are looking for maximum performance and battery life for your phone.

So, there is no good reason to force close apps, unless you are having a problem with an app or apps.