Like others said, this is a very personal opinion, so whatever you do, do what's best for
you; don't lemming which ever side gets more votes without testing, mock up, and more testing.
Me, I've gone straight on my 6 button CP (actually a 7 button SF/neogeo layout), and don't think I'll go curved if I make another. If I wanted a curve to match my natural bent fingers layout, the curve would be so small it would look like I was inept in drilling in a straight line and missed by a millimeter. I don't want to slant the buttons either. (My other CPs, with only two and three buttons, are slanted and curved, respectively, though.)
I like the straight for a few reasons besides the "bad workmanship" look of my fingers curve. Straight is easier to make.

I've found, based on my friend's bought CPs, my friends have different "curves" they like, and complain if the curve is "wrong" (even if "perfect" for someone else), but all can use straight
without whining happily.

Finding the buttons without looking at the CP is much easier for me if straight (for my friends, the "perfect" curve is also easy to find buttons, but not with the "wrong" curve). The "easy to find" is both for 6 button games as well as most other games in mame, including most two and three button games, and toobin (a 5 button game). IOW, the straight layout is most flexible and fits most games; even if not "perfect" for all of them, it's closer to "perfect" for more games than my friends' CPs. And straight matches the look and feel of the arcade cabs I played when I was a teenager.
FWIW, I have a few CP tops rather than Frankinpaneling: The above 2 joy 7 button CP, a 2 rotary joy 2B, a 2 TB 2B, a 1 TB 3B, and a 1 4-way 1 flightstick 1 spinner 2B CP. The two button CPs have slanted buttons (you can't have a curve with two points) at about 15 degrees from horizontal (each differs a little), and the three work fine: I do play 2 button games best with mostly straight fingers, unlike three and six button games. I'm not very happy with my three button curve (too much IMO, but friends don't think it's enough).
The one thing I think I'd change on my 7 button CP is to move the buttons farther up the CP. Right now the buttons are equal distance above and below the joystick; I'd move it a little higher so my left joystick ahoulder/arm isn't closer to the screen as much (same with my first 2 button CP).
Again, that's just my opinion, and I know others here feel differently. As others said, mock it up, test what's best for you, and have fun!
