I believe that it truly depends on the group composition and the player experience as well. I'm DMing a group of 5 people right now through an adventure in Pathfinder, however they are younger and without much experience in the roleplaying/pathfinder world. I adjusted things very little for right now and they are doing alright, and they are learning the different tactics specific to each type of character. If your characters have experience playing as well as have a decently balanced group (at least one type of healer, etc), then you might want to adjust the encounters a little bit.
Adjusting them by adding smaller minions to the encounters, is most of the time a good idea. It helps out the morale of the players as they quickly destroy the chump monsters, but it also makes the normal encounters harder because it gives more turns to the planned evil creatures because the minions are the first to go. (Magneto X-Men 3 "That is why the pawns go first" during the endbattle.)
I would probably upgrade the CR on encounters that you feel are significant. But in the end, you're the GM! If you feel after the fact that the encounter drained more resources then was planned (after you adjusted the monsters) give them more XP by upping the CR. A good GM needs to be able to adjust things on the fly, cause even if you completely prepare for an encounter, you never know what ideas can occur in the minds of the players, good or bad.
Also, unless you're planning to play the entire adventure in one sit down, you'll be able to see the power level of your players and be able to adjust accordingly! If they get destroyed in one encounter you adapted, then don't change a later one down the path and see what happens.
When you GM more, you'll get better at this!