When adding class levels to monsters they "receive +4, +4, +2, +2, +0, and –2 adjustments to their ability scores". If the monster is lacking an ability score, such as Constructs and Undeads both lacking Con, which adjustment should I drop? The –2, or is that somehow cheating?
Also, how do monsters with class levels gain feats and ability score increases? Say a monster has 3HD. I add a single class level. Does the monster gain an ability score increase, since it now has 4HD? Or does it gain a feat as a first level character?
I would drop the +0 as a general principle.
It is difficult for the GM to cheat, after all you can just add a +2 to a stat if you want to, in and of itself it is unlikely to alter the CR.
Monster HD do not count for ability score increases. but they do gain feats based on their total HD.
EDIT: Looks like I was wrong about the ability score increase - step 3 does increase ability score increases for every 4 HD added. I've somehow completely glossed over that for the last lots of years...