Ides is correct...for routine cleanings and as long as you are not having any major infection issues you should be fine w/o taking any anti-b's. If you have had recent surgery then you may need to take some.
The person to ask is the dentist...or your hygienist...your gastro is not going to be as aware as the dentist especially since the dentist is going to be more aware of what procedures you are coming in for.
I know all of this...not because I am genius...but because my wife is a dental hygienist (Has been for 30 years) and we have had this discussion several times...once or twice because of posts similar to yours on this site.
One or two other things...this is where I get up on my soap box...be sure to ask your hygienist if you should, because of the crohns or meds you are on come in more than twice a year. We all, because of this dd are at greater risk for teeth and gum issues. The steroids and other drugs can really play a number on our mouths. SECOND THING....BUT MOST IMPORTANT!!!!!! ask for them to check your cheeks and gums for signs of cancer....it is becoming more and more common....found early you stand a great chance of being okay....let it go and you could lose your lips, gums, jaw or worse your life.....
Final thing....Yes randy you were right also...YOU are special. lol
hope this helps