Beginners do tend to start on very soft reeds. This is probably because for the first few weeks of playing the amount of pressure required to create a stable tone might otherwise cause too much pain beneath the lower lip. However there is no need to think of it like weight lifting and have to build up to extremely hard reeds. In fact once you are able to play harder reeds, it can actually be a detriment because although hard reeds can make high notes and the altissimo range easier to produce, it's often at the expense of low notes and reduced dynamics.
At one stage I was playing #4 Rico reeds and harder, but then realised that with softer reeds I could get a much more versatile sound. However I found it difficult to reach the higher notes and to get a lot of power and loudness needed for some genres. The answer was to work on air support and breath control. In fact paradoxically it seemed that playing softer reeds was more difficult, but with enough work on tone production and breathing it's actually possible to get a much better range, tone and dynamic range using medium soft reeds.