How do I position roof trusses without using a string line?
How do you make sure you have a perfect rectangle - corner to corner will always be equal - make your measurement the third angle of a triangle. 

So for all intents and purposes your question asks how to do this with out a string line and your concern is because of the wind. 

A perfect rectangle will have equal length from one corner to corner measurement (South-East to North-West) as it will from the other corner to corner (South-West to North-East ). Because basically it is two right triangles joined together.

So if you start at placement with your first truss - you make one measure on one side after that it is simple. See my example picture - black lines are trusses , orange lines are wall supports and the light blue is a measurement you make - whether you want to use a single pre-measured string or tape measure - you are not doing this by dragging a line across the roof .. and this is easy enough with a laser line as well (maybe easier). 

You can place the first truss and measure where to put the second truss and cut a line at that point - or use the Pythagorean Equation A² + B² = C².

A² = Truss length.
B² = center on center truss to truss.
C² = my little blue lines distance corner to corner.

A laser will work fine and you will not have any huge setup time - this is easier than you think.

[If the math is too much use the scientific calculator on your computer.]