At what level of power remaining should my iphone 7 be recharged? Some friends say to let it run all the way down and others say it doesn't matter. What is Apple's answer?

With modern phones this is much less of an issue. Software on the phone takes care of the battery well.

It is typically healthy for the lifetime of a lithium ion battery to unplug your phone, let it drain to 85% or less before plugging it up again. Once full it should be unplugged. This is hard to do manually with any regularity or without inconveniencing yourself. Some devices are programmed to do this behaviour automatically, but I can't say if the iPhone does or not. You may try asking on the apple website.