I always refer to the concave as the 'smile' and I find it works better when I put the 'smile' on top. A few times, I found myself doing it the opposite way... it worked, but didn't seem so easy to me

Ok, I feel like I've learned so much trying everyone's suggestions. I've also ripped a ton of seams.
So first, the comment about cutting on the grain is Sooooo right on. I learned that the hard way last week, so no problems there for me.
Second, which curve goes on top really depends on which method you use. I find that when I use pins having the convex or 'smile' piece on top is the easiest... The same can be said if you slowly feed it in using a walking foot.
If you are using the 1/4"seam guide method or the curve Master foot method the convex or boob piece should be on top.
Third, as said before, using the seam guide or curve Master method marking the half way point is just good for my reference so I know if I'm off or not.
Forth, I Stinking timed myself using the 1/4" seam guide method for my inside curves (the little ones) from grabbing the fabric, lining it up, in the machine, sewing, and then sniping the thread, it took a total of 1 minute 22 seconds. I find that even with my tight curves that method Yields a perfect curve every time quickly and efficiently. For larger broad curves I still find pinning is the way to go.