Maaaayyyybe.
It depends on the day you're going, the time wishes is going off, if there are parties going on, how late the MK is open, the time of year it is... there are some times when you'll be able to walk right in to California Grill. Unfortunately, that's rare.
Five is just when it opens, so you can get lucky there (arrive around 4:40 and ask if you'll be able to get to the bar at 5; you'll probably get a yes): just be prepared to wait for everyone who has rezes from 5 to 5:30 to probably be escorted up before you so you may not get to the bar till 5:30 or so. It is definitely the best time to try to get in to California Grill without a rez, but keep in mind that a lot of people even with rezzes go up to the bar and have a cocktail/sushi/flatbread before moving in to the main dining room and that will be allowed. We've gotten there at 4:45 and been told that because of how full they're booked, they're not allowing anyone in to the lounge until seeing if there's a no show. And then there at times when we've gotten there at eight when anyone's allowed in. It really just depends.
Doing it on a night with no wishes (or a late wishes) that isn't Friday/Saturday (so fewer locals) will up your chances.
Personally, I'd just make a reservation and not chance it, but no matter what, good luck!