should my back tires be perfectly strait or should they point in a little ?

You talking about toe or camber?

i belive toe they there straight up and down

On some vehicles, SLIGHT toe in at the rear is good (steering is more stable).
I have NEVER seen toe out on the rear of any vehicle.

For the sand, it shouldn't matter as much as for the asphalt, but remember in measuring, that you must cycle the suspension and measure alignment at various points in the travel.

Absolutely straight is the most straight forward (pun only slightly intended) way and simplifies everything.
Are you welding at this point or wrenching?

welding

1/4 inch toe in on rear trailing arms = 1/8 per side

A simple way to measure this in set up is to bolt your hubs/wheels on and measure total width (widest point)

Clamp a 2x4 or similar of exact length you measured, centered to the front of the frame.

Tie 1 piece of string all the way around the car around both back tires and the 2x4 so it is tight.
Now look at the string on the sidewalls of your rear tires.

There should be a slight gap between the string and the leading part of the tire. 1/8" is a great recommendation.