"Some" bolts are designed to stretch a certain amount as the final torque is set. Usually these are one time use only bolts (or studs in some applications). Once this bolt has been torqued to final spec it is "used" and even if you did it two minutes ago and have to take it out to do something you forgot, it is supposed to be replaced.
Now to complicate things further, some bolts are "required" to be replaced any time they are removed. This is usually seen on things like brake calipers and differential locking bolts. Typically this is because the new bolt has a factory installed locking compound that would no longer be effective if it were to be re-used. I believe there are two reasons for this "requirement" from the manufacturers:
1) it increases sales of parts
2) it insures that even the dealership mechanics do things the factory way, cutting the liability tail between the dealership and the manufacturer.
For bell housing bolts, are they coarse thread and screw into a blind hole or do they go all the way through and have a nut on the other side?
What diameter is the bolt? (not what size is the head)
Take a look at the number on the top of the bolt head. It should be something like 8.8 or 10.8. This will indicate the grade (strength) of the bolt. You can then look up the standard torque setting for a bolt of that diameter and strength rating.
Keep in mind that most times when a bolt is threaded directly into cast iron it will be course threads and the torque values will be lower as the bolt is likely stronger than the threads in the casting. This keeps you from striping out the casting.
Do you "have" to use new bolts? Absolutely not, but you have to decide how important that bolt is and the cost of it's failure. If you do re-use them and the use of a locking compound is appropriate, just make sure you get the old compound completely removed and that you apply new compound to clean and dry threads. Also choose your compound carefully to match the application. In other words, if you need to heat it to break it loose the next time, can that be done without damage to nearby components.
You might check with a parts house to see if new bolts are available, what the price is and if they have pre-installed locking compound.