As you can surmise from the posts here, all approaches damage the amps and guitars and relationships while, OTOH, all approaches prevent said problems. Whichever side you're on, you'll find support.
I unplug from both instrument and amp when I'm done for the day but that's mostly because the plug gets in the way of the amp cover, which I use, and the bass goes into a wall-mounted rack, which is easier with no cable hanging from it. This is what I do and why; do whatever floats your boat, it does not bother me and I do not care. FWIW inserting and removing the plug keeps the contact surfaces clean and corrosion free. That's part of the design. I don't buy any reason except convenience for leaving a cable plugged in.
I like my Neutrik right angle phone plugs. I've used nearly all brands through the years since 1962 when I started in electronics. Since discovering Neutriks a couple years ago I've been cutting the others off my premium cables and installing Neutriks. I still unplug 'em.
The jacks in our amps and instruments are rated by the manufacturers for hundreds of thousands of insertions, so you won't be wearing them out unless you give up playing to take up poking and pulling the plugs. (e.g. Insert/disconnect once per second, eight hours per day, five days per week with no breaks will get you to a half million insertions in about a month. Bor-r-r-ring. The higher quality jacks are rated for two million insertions. You're going to damage them through abuse but you're mighty unlikely to wear them out.)