Shadowing can be treated like a checkbox item. Once you have enough, it doesn't matter. Just try to get over 24 with multiple specialties.
As for volunteering, you want to make it look as legitimate as possible. There's nothing ADCOMs hate more in this area than a box-checker. They also don't like to see people who got all their hours within a few months of applying. So here's what to do. Make sure you have over 100 hours when you apply, and DO NOT DROP THE ACTIVITY BEFORE FILLING OUT AMCAS. You want to make sure tht you've listed the activity as "current." It won't look good if you're not volunteering while you're interviewing. Once you have an acceptance in hand, then you don't have to think twice about dropping it.