Spars mainly have to resist bending stress. The greater a spar's moment of inertia, the lower the bending stress. Moment of inertia depends on the cross-sectional area and shape of the spar. A square spar has a greater moment of inertia than a round spar of the same area. Spars also have to resist compression. The greater the cross-sectional area, the more compression it can resist.

A 1 1/2" square spar has a greater moment of inertia than a 1 1/2" diameter circular spar. It also has a greater cross-sectional area. The square spare will resist bending and compression better.

The same is true for the case you're asking about, just shaving off the corners. I don't have time to do the calculations right now, but if you google "beam design" you can find the formulas and work it out yourself.

In the meantime, the safest thing to do is to stick with the designer's specs.

Laszlo

 Laszlo nailed it.  If you run the numbers, taking a square spar and making it round will decrease its resistance to bending by ~38%.  Here are the equations from the textbook from my graduate material class. 

I would agree heartly with Lazlo and Mark: build the spars as designed.  The mast, especially, needs to be really stiff with a lug rig, because it is under much compression between the upper and lower yards (boom, if you like).  Unlike a sail where the luff is attached to the mast all along, any bending of the mast between the upper and lower yards of a lug sail will slacken the tension vital to keeping the sail in proper shape and greatly diminish the drive, especially hard on the wind.  If you paint the top part of the mast white in contrast to the rest, it won't look so clunky.

As for the yards, a bit modest tapering at the ends could be a good thing, and helps them look less clunky.  We gave a bit more taper on the aft ends than the forward ends, which seemed to look right for our Passagemaker Dinghy.  With the standing lug of the Eastport (vs. the balanced lug of the PMD), I wouldn't taper the forward end of the lower yard (boom).  Stresses are different there.

White paint at the ends of the spars helps the looks, too.  See a photo of our PMD under sail which currently #9 of 66 in the PMD Takeapart gallery.  (Right click, open the image in a new tab or window, and enlarge for a better view.)

.....Michael

Well... the consenus appears to be that the least work (round over on a router table) and sticking to the design is the way to go.

Laszlo and Mark_N, thanks for the technical explanation.
It made me want to figure it out, which made me realize that it's not quite as simple as a square and a circle, since we're really talking about a rounded square and a circle. But I gave up calculating for the rounded square when I further realized I'd have to take into consideration centroid offsets from the global centroid for the sub-areas, or something to that effect. I decided it was enough fun with math for one day.

Michael, thanks for your explanation that was easy to visualize. And for your suggestion of the white paint.
The white tipped mast me that I had read on the forum here that you have, or had, a Menger 19. So do we! We are still pretty new to sailing have bought it last year. I don't think we could have made a better choice of boat. We sprung for the upgraded tanbark sail for the ENP to go along with the tanbark on the catboat, so a white tipped mast on the pram will will be even better!

digger