I waited until about my third year before I put in a yellow pages ad. I wanted to try to establish myself a little bit before I jumped into it big time. But I do admit, yellow pages ads work great for me.
a few things
1. You may not have the equipment or employees to have the work sent your way or you may have to turn down people
2. You might not be able to afford it
3. You decide that the business isn't for you, and you just wasted a years worth of advertising and money
4. You change phone numbers