You need to build a v shape from the bottom of the rail to the stringer. If you put flats next to the rail you’ll end up with a spine instead of a nice roll or v. Instead of making the board easy to roll from rail to rail you’ll have a board the wants to track more than roll.
If you put the bottom contours in before you cut your deck rail bands, you can adjust the rail and tail thickness from the top.
I’ve watched a few guys cut v’s with power planers and I bow to their skills. I’ve seen them cut like rail bands and I’ve seen them cut from stringer to rail (that version is just plain nuts). A disk sander with a hard pad seems best to me but I drink too much coffee… I do mine by hand using a hand plane and sanding blocks. Here’s how I do it.
Start with a roughed, blocked shape. Outline cut out and trued. The tail rocker should be a little flatter than you want the final version to be by 1/4 to 1/8" (more if you are really planning on a deep v).
If you look at the board from the side, you’ll see that the rail rocker is exactly the same as the stringer rocker. If they are not you have a twist in the blank or didn’t block it correctly. Fix it now or you will be sorry later. Keep the blank with the bottom up for some measuring…
Take a pencil and mark a dot on the outline cut right where it meets the bottom where you want the v to start. Then put another dot where you want to to have the maximum depth of v. Measure down from where the bottom and outline meet and put that dot at the maximum depth of v you want to cut. Then you put one last mark where the v should end.
Those three dots now mark the rail rocker. You can add some more dots or take a batten and connect them up into a smooth curve. This will give you a guide to cutting the v and help keep your rail rocker nice and smooth.
Do the same dot thing on the other side. Try to keep everything as squared up as you can to keep it all symetrical (unless you’re an asymetrical guy…not that there’s anything wrong with that!)
I start cutting a 45 degree cut where the bottom and the outline meet, like a mini rail band. Make sure it all flows smoothly, feathering in from the front of the board and feathering back out at the tail. Just keep knawing away until you get to the stringer.
When you’re done, you should find a funky spot where the v ends at the tail. That is where you’ll add that 1/4 to 1/8" kick to the rocker to smooth it away.
I always refered to this kind of v as “dome” v. You can make the v panels flat for a crispy feeling or round for a smooth feeling.