Do I need a mounting kit to install the Pioneer AVIC-X910BT in a 2007 Nissan Frontier King Cab (standard 4 speaker option)?

Typically you'll need a kit of some kind to do any type of install.

This isn't your standard HU that comes with your car. To make it as "factory fresh" as possible you'll probably need several things if you're doing it yourself.

You'll need:

Dash Kit

Antenna Adapter

Harness Adapter

Time

Patience

Google

Usually particular to your year/make sometimes to just make, the kit helps everything look like it really belongs. Unless your doing a custom car with a custom look at which point your SoL.

The antenna adapter is b/c sometimes Nissan/Honda have antenna jacks that don't fit up quite right

When you remove your stock radio it typically has a connection point between it and the car. These bundles of wire hook up to your radio..but don't be a n00b and cut the wire inside your dash. Use a wiring harness adapter and your 910's harness to wire everything up (don't forget the bypass)

Just google your car and you'll probably find a forum dedicated to all things Nissan Frontier King Cab and they typically have an "audio" forum to look through. Any modifications (like radios/amps/subs) will be in there and sometimes they'll even throw in a Do It Yourself Guide as well as tips/tricks that they use.

Take your time. Especially if this is your first rodeo. You'll learn a lot...and if you're not careful you can break something too....