If by "safe," you mean a G.P.A. with which you're assured acceptance, there is none. Even with a 4.0, you might lose your seat to a student with a G.P.A. of 3.6 and plenty of experience shadowing and volunteering, and a well-rounded academic record (say, all you've done is complete the pre-reqs. for optometry, and completed the requirements of your biology major). If you mean, "What's a strong G.P.A.?" the closer you are to 4.0, the better...