Prague restaurant reservations necessary?

They are only necessary if the restaurant is full.

We spent a month there, and when we ate out we only made reservations once.

I am a regular solo traveler to Prague most recently a couple of weeks ago, never had a problem, if I am with a group of people then we sometimes book via a restaurant website or pop in if we are passing and book for later that day. but solo or couples there is seldom a major problem. On my last trip I on several days had a large lunch and in the evening either bought a snack from one of the numerous carry-out places and on one evening just bought a couple of snacks from Tesco's

You would only need reservations here if you want to guarantee eating in a certain place i.e. maybe a place has a reputation for a special meal or for a scenic view etc. If you are happy to just turn up then restaurants should be happy to seat you at a table for two. Decent restaurants will take early trade but they'll make it clear that you only have an hour etc before any reservations kick-in. Try and eat off-peak i.e. if you can be finished for lunch before 12pm and for dinner before 7pm you'll have a lot more choice of places. Sometimes people are happy to queue for a table. I see this quite often outside Bistro Monk in the Old Town. You'll find not many restaurants are using any booking systems unless they offer their own option through their own website.