The undesirable effects of feeding too frequently without an interviening bowel movement include: vomiting, impaction, prolapse, obesity and resultantly, suffering and death.
The "no more than 3 meals w/o BM" is a good one. The rule is of course ETB/GTP lore, and if violated on occasion, your snake won't instantly croak. However it is a good rule. I use a log to record feedings and more importantly, BMs. With the rest of my collection, I am much more casual; if I thaw out too many rats, I can always find somebody who can chow down on the excess (but I try to alternate pigs). I would never play that game with emmies.
The shower advice was good and it worked for you. Like other snakes, exercise will help in this regard too. Tree boas are way different than other snakes in many cool ways, but they become pretty simple with time. Just don't overfeed.