Since the 1+ has an SS output stage, it isn't really necessary to plug and unplug the headphones.
That said, if you think that you might eventually get an all-tube amp, this is an excellent habit to develop. If tubes are going to arc and short-out, it typically happens the instant power is turned on and/or off. And since the amp's power tubes are connected directly to your headphones, very bad things can and do happen. And further, to save wear and tear on the amp's headphone jack, I have a 2-ft headphone extention cable always plugged into the amp, and then connect and disconnect the headphones from the extension cable. But again, it isn't necessary with the 1+, but still, it is a good idea.
Oh, but see, this is what I normally always do. Power things up with volume all the way down. Plug in headphones, listen, volume down, unplug, power down. Of course, i don't bother with this with the iPhone, but with Duet or my Korg and any other audio gear. Don't want power up or down transients on my headphones!
BUT, this is the opposite of what the LD users manual says and the opposite of what other posts in this thread have claimed. Connect enerything, then power up the LD. I don't like this. Plus, if you want to switch headphones, it's a royal pain. Apparently it is not only not necessary but is bad advice to boot.