If shadowing or volunteering are going to get in the way of grades or adequate DAT preparation, then yes, cut back. With few exceptions, volunteering is icing on the cake. Make sure you have a decent cake (grades/DAT) before anything else.
As for shadowing, the vibe on here is 50-100. Look into your schools of choice to see if they're one of the special ones who have a strict required number of shadowing hours.
When you apply, you can leave the end date open for volunteering and shadowing and just put what hours you have if they are indeed on-going.
I advise you have 100 hours of volunteering and 100 hours of shadowing. Like the above poster suggested, do not allow volunteering and EC to get in the way of studies. Your numbers matter more than EC. However, many dental schools REQUIRE 100 hours of shadowing. It's important to know dentistry is what you want to do, so shadowing should be something you look forward to. It shouldn't be something you have to do to rack up hours.
Remember, you must enjoy your job whatever it is. Shadowing is very important for this reason. It also comes up during interviews. If you have great numbers, but nothing to talk about. I'd imagine an interview would be slow.
NEVER go into anything for money-especially medicine or dentistry.