I've always used the 2X4 method described on Sheldon Brown's site to spread the rear triangle, but I thought I'd try using a threaded rod, nuts and washers this time because I had the hardware on hand. I needed to spread a mixte frame from 126 to 130. Mixtes are a little harder because they have three stays per side.
I had the frame and fork in the workstand hanging by the seatpost. The first problem I had was that the threaded rod tended to turn and slide out of the dropout when I turned the nut, so I had to hold it in place with my left hand. The second problem was that, in order to check progress, I had to back off the nut and that's a lot of work when you're turning the nut 1/3 turn at a time with an open-end wrench with one hand and gripping the threaded rod with the other. I ended up taking it all the way out to 165mm, but it sprung back to 127mm when I backed off the nut.
It always bothered me that using this method assumes that the stays will bend equally when equal pressure is applied to them. There's no reason to believe that, especially when the drive side stay is crimped for chainwheel clearance.
I had just decided to give up this lame method and do it the right way when the seatpost pulled out of the frame. I had a firm grip on the threaded rod and I instictively raised my arm to keep the frame from hitting the concrete. The frame swung down and a fork end caught me on the shin bone. After I got through screaming and swearing, I got out the 2X4, string and ruler, laid the frame on a piece of carpet and spent about five minuts spreading a realigning the frame the right way.
The reason I have time to write all of this is my lower leg and foot swelled up and turned purple and I'm under doctor's orders to stay home from work and off my feet. I'm waiting for UPS to deliver a bunch of parts for the mixte, so I won't be staying off my feet.