One admissions director from a top med school said PLS don't shadow especially if you already have had clinical exposure/volunteer experience in a hospital setting. He said he's sick of seeing a gazillion applicants "shadow" and everyone basically be the same kind of applicant.
What I took away from his comment and just general research into all of this shadowing/volunteering business is you just need to confidently show, through whatever experience (whether it's volunteering/shadowing), that you KNOW what it's like to be a doctor.
If you feel like you can intelligently and convincingly show the adcom that you know what it's like a doctor, through your hospital volunteer experience (and that shows through on your app and in your interviews), you're fine.
Is it "safer" to have some shadowing experience? sure... but I definitely don't think it's really really necessary if you think you've had meaningful exposure elsewhere.