Paul Heaver wrote an article about how important the placement of asteroids can be when playing your squad or defending against your opponents squad. His "Turn Zero" articles are a must read for tournament level playing.
http://www.fantasyflightgames.com/edge_news.asp?eidn=4709
Here are some asteroid placement strategies outlined and what benefits they convey when you use them. I'm using the asteroid deployment zone shown on a 3x3 mat in Vassal.

The Asteroid Deployment Zone is the center square area. Any edge of the center box is range 2 to the mat edge. Range 1 deployment areas for ships are top, bottom, and both sides for a side to side match.
In every sample your ship deployment area is the one on the bottom and the opponent is coming from the top.
The Laneway (Aggressive)


Place one asteroid inside range 3 of your one corner (range 3 from both mat edges) where you want the laneway. Place your next asteroid range 2 (measured from the back edge of the first asteroid) and range 3 from the same mat edge. This blocks any asteroids from being put between your asteroids and the edge of the mat (note the 3rd asteroid cannot be placed where it is due to the other two being to close). This laneway allows for a wider area, clear of obstacles and a quick breakout with lots of room to maneuver. Works good if you have mid to high PS and/or you have ships with boost.
The Anti-Phantom (Defensive- Corner Castle Strategy)


So you forgot your turret in your other fleet and are now facing down a Phantom, one of the hardest ships to pin down. This asteroid placement can help. By using the far corner you stack the asteroids in a way that can block decloaking possibilities, limiting the options on that phantom and allowing you a shot and bringing your arcs to bear. Place your first asteroid in the corner of the asteroid zone and then each additional asteroid at range 1 on top of your corner placement and on the bottom of the deployment zone, also at range 1 of the corner asteroid.
In this image the side decloaking lanes are blocked by the asteroids or are off the mat, leaving a very predictable straight decloak available. This strategy works if you have ships that can all K-turn; It is not possible to stay in the corner if you don't have the K-turn ability. Works best if you face and fly parallel to your edge at the start.
Pro-Swarm (Aggresive- The Lineup)


Setup asteroids as close to your edge as possible and in a line. This allows you to maneuver past and gain access to the center, where you'll be able to turn in and confront your opponent easier with less asteroids breaking up your formation.
Setup in one side and deploy each asteroid as close to your deployment side as possible. Rocket past your own asteroids and go for the kill en-mass.
Anti-Swarm (Defensive - The T-Pee)


Your opponent brought the numbers and you need every advantage you can get if your squad is going to emerge victorious against his swarm.
Your first asteroid placement should be near the center of the table. You want to obstruct the middle area as much as possible. This helps to break up his numbers, causing them to lose actions and possibly take damage as he tries to engage you. Divide and conquer is the way to go for the win here. Pick off the injured and the lone straggler.
Your 2nd and 3rd asteroid placements should be at a 30 deg angle from the center one. The maneuvering options in X-wing are 45 or 90, so the odd 30 deg angle placement makes it harder to maneuver ships through or forces them around.
Your game can be won or lost from the moment you place your asteroids so understand how asteroids can help you or hinder you, depending on how you, or your opponent, are going to fly your squads.
Fly Casual.
Edited by Sergovan