Invariably, if you ride enough, for long enough, you will be caught out riding in low visibility conditions (darkness, rain, or both...or something else, too).
Thus, the more lights you have, the better. Up to what your the rest of the bike's electrical system can comfortably handle, of course.
Since our bike doesn't have a lot of spare electrical capacity, LEDs are of course something of a "silver bullet" for us.
With all that in mind, I installed one of various LED light kits available now, thinking that it would make a nice set of marker lights, supplementing the headlight and driving lights I already had.
After having done this, I realized that LEDs and cruisers are basically made for each other (mostly due to the presence of chrome or similar finishes).
Basically, the limit here is your imagination. I installed LED strips (of an appropriate length) under the fuel tank, under the battery box, underneath the saddlebags, on the front of my windshield, on the front forks, under the airboxes, on both sides of the swingarm, etc, to make the bike stand out as much as possible in traffic. These LEDs show up well even in daylight (such as when it's overcast).
LEDs underneath floorboards and/or saddlebags create a really nice "ground" effect that can also give your bike a distinctive look.
I would encourage you to "think outside the box"...and test the location & look of each LED by using post-it-note/wall picture putty that will hold each LED in place temporarily, to "fine tune" the look you want. Stay away from colors that will attract the ire of law enforcement (e.g., blue--& no flashing colors while the bike is in motion). The best color to work with is probably amber, or yellow...if visibility in traffic is your goal (green is probably not a bad idea either for this).
Also--check the laws in your state to see if amber on the back of your bike is legal--or if you need to stick with just red there.
This is true for working on a bike (or anything else) in general, but don't undertake a project like this unless you have a covered shelter where you can quit a project like this and come back to it later...and your stuff will still be there when you get back.
Hope some of this info helps....