You can name the sheets directly in the IDE. The defaults are SheetX. Select the sheet in the IDE and rename it in the properties window. You can then use these names to refer to the sheets in code:
Sheet1.Range("a1").Value=x
Regarding your question though I would say 9 times out of 10 it's probably better to do whatever is easiest to code! If you are using them alot then declaring global variables will makie things much easier to read and edit at a later time. Make sure that you destroy the variables when you have finished with them though by setting any objects to nothing after the last use.