You should start shadowing as soon as you can. You'll want to have around 100 hours, so it is much easier if you spread it out a bit. I didn't ever have any dentists sign anything, but I did keep track of the dates and hours on my computer. This made it pretty easy when filling out that section of the AADSAS application. None of the schools I applied to ever asked for anything else.
I started Freshmen year of college and continued throughout. You don't need them to sign anything to prove that you did but you should keep a note of your hours on your own and be honest about the hours you shadowed. After all, who wouldn't appreciate an honest dentist?
lol the order is to shadow THEN to get a *light bulb * and say "hey I wanna be a dentist!!!"
But seriously, you should do shadowing before you apply.... If you do your shadowing after you apply (or during)... some interviewers might ask you "how did you know you wanted to a dentist without much shadowing"
And even some schools will require the dentist you shadow to put down how many hours you shadowed in the LOR (I can't remember, I think temple does this, they require 30 hrs)