Drop out alignment will only make the drop out surfaces parallel to each other. This is needed to insure that there are no extra stresses placed on the axle/bearings. depending on the axle, bearings and frame there can be some bending to the axle in the drops are not parallel. This bending can have secondary ill effects. Drop out aligning tools are inserted into each slot and have surfaces that locate close to each other and allow one to visualize how they relate to each other. You can make some easily enough from a spare QR axle with cones and lock nuts. Cut the axle in half lengthwise, thread on the cones enough so that the axle halfs will fit into the slots with a little bit extending outside the drops. Thread on the lock nuts. What you now have are two hollow tubes that almost but not quite touch each other and each is square with their drop out. By sighting through the hollows you will be able to see how each axle tube aligns with the other. You can slide in a skewer to see how it goes into the further tube. When the drops are parallel (making sure the axle tubes are in the same spot in the slots) you'll be able to sight through the two tubes completely or be able to slide the QR skewer through the tubes with out it's hanging up or having to bend a bit. To do the actual straightening you will likely want to remove the axle tube and use either a large crescent wrench (making sure to engage the entire drop out axle surfaces) or a all thread rod with washers. Using the axle tube might distort the lock nuts as they are usually fairly thin.

One aspect of spreading that I don't think has been mentioned (I may have missed it in another reply, sorry if I did. I hate repeating other's comments) is that simply spreading both sides of the rear triangle at the same time will not insure that it stays in line with the main frame (if it was actually in line to start with). many frames, especially lower cost or older ones) have the chain stays dimpled for tire clearance but only the RH one for chain ring clearance. So it's not uncommon to have the RH stays move sooner and further then the LH side.

If you brought in the frame to my work to have it spread we'd charge about $35+ depending on factors yet to be discovered. The frame would need to be fully stripped for the lower end of the cost range to apply (it's easier to do with the BB out). And we'd want the wheel that was going into the frame with it. If you did the spreading and only needed the drop outs aligned then the bike could be built up with out effecting the cost, maybe about $10+. Andy.