The world itself cannot be frozen, time will still go on as there will always be at least a few chunks loaded. But for the most part, everything is unloaded, thus, server shouldn't have any issues as long as machinery that still operates even in unloaded chunks (like Industrial Craft Nuclear Reactors) don't cause some other...side affects.
I think it depends on how your server is hosted. For example, my Fragnet server has an option called "sleep mode" where if there's no players conencted, it goes into a semi-shutdown state. In this state, it conserves on using RAM and other things until someone connects.
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"The Universe is trying to kill you. Outsmart the Universe."
--Neil deGrasse Tyson