I would say it is safe during the day. I have been on 116th in the evening and had no problems. If you are concerned, you can take the M116 bus which drops off near Costco - 116th and 1st (also, it does stop a 116th and Pleasant Ave after it loops up to 120th St).

http://www.mta.info/nyct/bus/schedule/manh/m116cur.pdf

Would love to know your impressions of the Costco? Was going to check it out, but may wait til the Target opens there (or so they say).