I did a 21" on my 15 RGS. LOVED the look but hated the fact that the skinnier tire caught all the ruts in the road. I also lowered the front an inch.

 

21 on the front and put an 18 on the rear so it evened it out, did not lower anything. Rides and handles as good if not better than stock. All you need on the front is a fender relocate kit, 2 small brackets that raise the fender up about an inch.

 

Here's my FLRT with a 21" twisted spoke wheel. All good. Did not lower the front end or raise the back (yes I know it's a trike and completely different than two wheeled dynamics). Fender just went into paint.

 

Front lowered 3/4" Before it was lowered it was a little wobbley on SLOW parking lot maneuvers but felt more solid on the hwy.

 

First, I want to thank everyone who responded for their suggestions, experience, and comments regarding the 21" front wheel. I did some additional research and called the company that I am purchasing the wheels from (Southern Machine). The salesman at SM told me from his experience, he did not feel that I would need to lower/add any kits to the front end. He said with the set up I have ordered, to include a front tire, the ride height on my RG would only be raised by 1/4".
I am going to try this set up with the stock front end and go from there. Also, speaking to the techs at the HD dealership, they did not see the need to add any suspension components with the 21" wheel.

Thanks again to those who responded. I will post pictures when everything gets installed.