You said "places I go" and "store". The one "famous" example I can think of where Visa has not been accepted but Mastercard (as well as AMEX and I think Discover) has been was with online tax payments for a few years (tho I think now finally Visa is accepted for those as well).

But that's just acceptance. There's a separate issue of promos. If you stay at hotels often, but aren't super loyal to one brand, be aware that sometimes one hotel chain will have a promo such as "stay two nights, get one night free" that only works with a Mastercard, then another chain may have the same kind of promo, or a promo such as double miles and/or points, that only works with a Visa! So if you're into taking advantage of such offers, it can be useful to have both a Visa and a Mastercard (tho if one of them you only use rarely, just for such promos, obviously get a no-annual-fee one for that).