Like 4gsr said, never seen one cut perfectly square. I've never seen a horizontal bandsaw that cuts truly square either for that matter. I've seen some that get close, but none are perfect. There are too many variables that can affect how the blade passes through the stock. Additionally they were never meant to cut perfectly square. They rough cut the stock so that it can be trued up on a machine better suited to the purpose, like a lathe or mill.
Now if you've got one that is going really wild, then you need to start looking at a number of things.
Blade - are you using a power hacksaw blade or a hand hacksaw blade? Hand hacksaw blades generally will not take the vertical load of a power hacksaw very well. They tend to twist in an attempt to flip over. Once a blade has been twisted, you can never get them to go back it seems. How sharp is the blade? Dull blades seem to make the crooked cutting problem worse.
Blade frame - the frame usually slides back and forth on a set of ways of some sort. Some saws used a simple piece of straight bar stock, while others used dovetail ways or anything in between. In most cases there are gibs that help the frame to run straight on its mating or guiding surfaces. Make sure those gibs are in proper adjustment if they are present and that they are not worn out. They also need plenty of oil.
Main Frame - Sometimes the pivots on the main frame are worn, allowing the saw frame to come down at a crooked angle.
Crankshaft - Crankshafts that use bronze bearings will run crooked if there is significant wear, causing the saw frame to run cooked as it is pushed back and forth.
Vise - make certain that the vise itself sits square with the blade and its travel.
Look for parts that do not sit square with the rest of the saw. You never know what may have gotten bent on a saw in its past history. Use a known good square too. Estimating what is square never works very well.
One thing that can give you a clue where to look is to determine in which plane the saw is cutting crooked, or if it is a combination of planes.