What is the recommended method of insulating PEX piping?
The "heat tape" you are referring to is what we use in Western Maryland as the "go to" way to keep our PEX piping from freezing.  It works great and is a low cost way of making sure your pipes don't freeze.  Warning though, make sure you have a way of always knowing the tape is working.  If you bury it behind a wall totally and can't check it a few times during the cold season, you run the chance of it malfunctioning.  You also have to make sure it does not overheat by overlapping the wire on itself.  And if the power goes out and it's below freezing, it also won't help at all.  Our company has rentals that are very old homes and apartments and the piping in some of them are very close to the exterior surfaces of the units so we use this all of the time with great success.  When we are using heat tape for plumbing behind walls and under floors, we use a product called Hotline internal pipe heaters.  This goes inside the pipe rather than outside.  It also prevents overheating that can cause a fire.  As I said before, if you bury regular heat tape behind something you have a huge risk for malfunctioning which can be anything from a non working tape to a fire.  Also, if you can get another type of insulation around the pipe, such as spray foam, that should do the trick.