There's got to be a bazillion ways to insure they're straight.  Assuming a straight hinge line I'd do it by making sure they were square to the hinge line (or offset by my desired amount), then making sure the stab is square on the fuse by the usual triangular measurements (you do make sure of this, don't you?).  If for some reason there isn't a straight hinge line, I'd draw a reference line and make sure they were square to that and proceed as above.

Speaking as an engineer with a long-time interest in aerodynamics, I can authoritatively quote numerous folks who actually know this stuff and say that yes, absolutely, it does make a difference.  Just like end plates on a wing, or winglets, they will slightly increase the effectiveness of the stab/elevator as a lifting system, and done right they will reduce the induced drag.

Speaking as an engineer with a long time to develop cynicism about nearly everything, I can also opine that whether they make enough difference to matter is a matter of conjecture, and whether the difference is positive is a matter of getting the details right.  While the induced drag at high elevator deflection will almost certainly be lowered, the interference drag from the extra surface intersections will be higher.  Further, when you have movable surfaces working against end plates like this, the necessary gap width to gain any advantage can be incredibly small; make it too big and you don't get much improvement in induced drag, you lose your extra lift, and the interference drag goes way up.

History tells all, and history tells that a lot of planes have been successful without them, and a lot of planes have been successful with them.  I think whether you use them really falls to aesthetics and personal preference.

Done right twin rudders can be awfully nice looking.  I've already staked out a position as a doubter for any stunning aerodynamic benefits, but that doesn't keep them from looking darn nice on the right plane.

Don't forget they also tilt in to the top.  I think the plans show a jig for that.

Just checked the plans, from a Brodak kit.  I dont see anything about tilt in at the top.

I need to make a correction here.  The Brodak Olympic plans do show that the tops of the rudders tilt in at 8°.  The rudder tilt is also mentioned in the instruction book.

In theory they act as end plates and alter the vortice pattern at the tips making the elevator more effective thus allowing a slightly smaller surface to be as effective as a larger surface.This is my understanding of them anyway. Oh, and put Randy's "Olympia" on the list of stunters with small tip plates on the ends of the horiz stabilzer also.

As another Olympic builder, I can say the fins tilt in at the top by design. Why? Gialdini says in his 1963 Olympic article that it looks better that way! Plus, artists and architects will tell you the fins would probably appear to tilt away from each other if actually vertical; the world's greatest example of this is the Parthenon, where all the columns tilt slightly towards the building center and would meet at a point far in the sky if extended. No straight lines anywhere in that building. Very apropos to the "Olympic." Anyway, I used a jig and a disc sander to machine the stab ends to the exact angles needed. I believe mine are not offset at all, but Gialdini's build article calls for 3 degrees offset.