If you ride fixed gear seated up every hill you will develop a pretty good base for when you actually want to stand up in the saddle and put down a lot of power.
But the more typical advice these days is get in a good spinny gear and pace yourself. Like Judd says, it depends on what kind of hill you’re climbing and how long you’re riding for.
Of course if you go far back enough (1903-1906), fixed gear was what everyone used in the Tour de France!
http://hizokucycles.bigcartel.com/single-speed-history
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