Once you've decided which agencies you'd prefer to target don't hit on them until your book is the best ever. Showing it to agencies you may not have considered readily will help you improve it. (Check out the agency list here on Ad-Mad, they're all good agencies).

It costs nothing to see people, the experience can only be valuable. You'll get useful criticism and as the industry is such a small one - creative directors all know each other and may refer you to another. It could take between 50 and 100 interviews before you get your first real job. And that's if you are lucky. All creative directors have a valued opinion (they didn't get there without winning awards) and can be very helpful. Almost all creative directors are nice people who want to help you.