Unless the scenario time limit is becoming a problem, you can rarely be too cautious.

Recon, recon, recon.

You want to go slowly. Only move a few units at most while considerably more are on overwatch for any fire they draw.

Once units have moved, they will see more if they can be still and spot over 2-3 minutes.

Look at the map hard and think about where you need to go, how you can get there, and where they might be waiting to stop you.

Never send a team anywhere a scout hasn't been.

Never send a squad anywhere a team hasn't been.

Never send a platoon anywhere a squad hasn't been.

Never send armor anywhere infantry hasn't been (unless your armor is leading infantry across open ground).

You'll always have to adapt to the circumstances at hand, but these general thoughts should serve you well.