Battery: should I turn off my Ipad before going to bed? or is it less damaging not to turn it off ever?

There is no technical reason to shut-down your iPad overnight - but you can do so if your choose to do so.

Perhaps consider that with the iPad fully shut-down, this will inhibit:

• all background housekeeping processes from running
• automatic backup to iCloud
• automatic retrieval of email
• sounding of notifications, alerts and alarms

If not intending to use your iPad for an extended period of many days, it may then be worthwhile to shut-down the iPad. Otherwise, doing so serves little (if any) direct benefit. Your iPad is designed to be left running continuously, recharging as and when needed.

As for preserving your iPad’s battery, minimising the number of full recharge cycles will help to prolong the overall battery lifespan.

Batteries - Maximising Performance - Apple

All batteries have a finite service life - with a limited number of charge/discharge cycles. Every battery charge cycle diminishes the available battery capacity by a tiny amount; this effect is cumulative - and causes the battery to slowly lose capacity over its usable life. It therefore follows that minimising battery use by using external power, where available, will prolong the overall battery lifespan. 

Apple quote expected iPad battery capacity to be 80% of the initial total battery capacity (when new) after 1000 full charging cycles.

A full charge cycle is any combination of charging that totals 100%. Unlike older battery chemistries, fully discharging and recharging is not beneficial - but instead increases the risk of the Li-ion battery being left in a discharged state - leading to unrecoverable battery damage (a phenomenon known as “deep discharge”).

The iPad is designed to manage and maintain the battery during charging. Leaving the iPad connected to the Power Adapter for extended periods (such as Kiosk applications) will not adversely affect the battery.