To compile or not to compile the system

[Vittorio]

Using cvsup, after having issued "make update",  it takes a long time to 
recompile the OS sources and kernel  by means of 
make buildworld
make kernel KERNCONF=MYKERNEL
and then..
mergemaster -p
make installworld 
etc...

If I frequently update the OS and ports this procedure becomes time-consuming 
indeed. 
Is there any way to know in advance if it is necessary to comply with the 
described procedure or if it can be skipped because there weren't changes 
between two close updating?
Ciao
Vittorio