I am one who believes in making 400 ISO the starting point for non-flash photos indoors, and also for bounce flash photos indoors, I usually suggest ISO 500 or ISO 640 as the "NORMAL" ISO value for bounced flash. For a number of reasons, but basically because starting at ISO 400, and not 100, and not 200, means there's already a two-stop, or at the least, a one-stop "safety cushion" that gives extra depth of field, extra room for error in focusing, and also makes the ambient light register more on flash shots and, even more-importantly, allows the flash to do its work without exhausting the batteries so quickly.
Since the 50D is a crop-frame camera, the 28-135mm zoom will give you "normal" to telephoto effective focal length settings, so, once again, ISO 400 is your friend indoors.
If you have a flash unit, I would shoot most of my flash shots at f/5 or f/5.6 on closer-range shots. Be aware that in big rooms, a flash does not "carry" all that far.
The best tip I can give is this" you have a d-slr, so you can SEE what is working immediately. Figure out what works best for the specific room or venue, meaning ISO, general focal length, and general lighting, like say, bounce flash upward at 30 degrees or whatever, and then work that successful pattern over and over. If for example, there's a nice, light-colored wall, and you're assigned to shoot casual posed shots, maybe bounce the on-camera flash off of that wall, and have 2's and 3's and 4-person groups form loose standing groups, and then shoot 20, 25 groups ALL in the same area where you can get what I call "a good bounce" off of the wall.
Use the d-slr as an asset. Get the ISO up to 400, or a bit higher, and WORK IT!!!!!!!!