Do I need to change cruising altitudes for really short legs?
As you are using the term flight following I'll assume you are in the US. Flight following is an advisory service, you tell them your intentions and they give you any traffic information you may have. If you are changing altitude or course you tell them, you do not ask permission because they cannot give it.
FAR 91.159 clearly states what altitudes you should be at using the semi-circular rule:

Except  while  holding  in  a  holding  pattern  of  2  minutes  or
less,  or  while  turning,  each  person  operating  an  air-craft
under VFR in level cruising flight more  than  3,000  feet  above  the
surface  shall maintain the appropriate altitude or  flight  level
prescribed  below, unless  otherwise authorized by ATC

So if you will be flying 3000ft or more above ground level you will be expected to fly the semi-circular rule, there are no exceptions made except those listed above. From a practical perspective you'll spend more time climbing and descending than at the other altitude so I think it's a fair question, the answer is to make the altitude change.