Is it practical (or is there any reason not to) upgrade components on a bicycle when they are replaced? For instance, I've got a 10 speed cassette on a tour bike that needs to be replaced. The bike is mostly Tiagra components, but is there any reason (other than cost, but in this case it seems like it's only about a $35 difference) to not upgrade to 105 or Ultegra? (Staying within the same 10 speed part.) Are there certain components that have to be replaced as a set if I upgrade?
I have looked at this compatibility chart, and if I understand it, it is saying that an Ultegra ST-6700 is a drop-in replacement for a Tiagra ST-4600. But I'm not sure if that is everything I have to look at, can I just do a drop in replacement of any component in any box?
I'm also concerned about whether there could be other issues with mis-matched components, for instance will a better component in one place lose some of its benefits or be degraded by a lower grade component that it interacts with?