I was about this ask this question in a comment from a different question of mine, but I thought it deserved its own question since it may help a fellow novice gardener.
How often should I loosen the soil in a vegetable garden? Do I want to loosen it deeply (12" to 24") before planting? Should I loosen it at all after planting? Should I do it in the Autumn once the plants have died to get ready for next year? Or should I simply do it a week or two before planting?
Once, and then don't walk on it. Since you are starting with raised beds, which presumably have not been walked on since they were made, I'd not get overly concerned about it.
There are, to be sure, contradictory opinions on this.
Some folks like to stir everything up - it feels like they are doing something. I'm presently of the opinion (having done the tedious double-digging thing with no particular effect/benefit) that the approach of leaving the soil as undisturbed as possible/practical and allowing the natural processes that are upset by enthusiastic tillage do their jobs is at least as good, and perhaps better.
If you are not walking on the soil, then roots and earthworms should provide all the loosening it needs. Feed it from the top and let the worms worry about moving that downward. If you think you have some sort of major compaction, you have on the one hand the option of mechanical approaches, and on the other using things with strong roots (such as diakon radish) to break it up.
Unless it is rock hard you really do not loosen the soil, because when you do you should keep in mind all of the weed seed under will be giving them a chance to sprout (unless it came in a bag). Otherwise I would just do it to you needs. I till the top every 2 weeks.
I am in the process of forming an opinion as well and have heard both sides of the story. As indicated in the previous responses, leave it alone after loosening it once. There is also a technique called double digging that has been popularized by John Jeavons (http://communitycrops.org/education/double-digging/) and you can read a lot about it online and watch videos. I think you will have to form your own opinion on this one and follow it.
