Let the kinds of pictures you take determine the answer.
Years ago, with a film camera, I took thousands of photos using a Canon 100 mm f/4 macro lens. Not a single one of those in the macro range was taken with flash. The film speeds varied from ASA (=ISO) 25 through 400. They were primarily outdoor shots of flowers and other small things (insects, etc.); the other main usage was portrait photography (100 mm is great for that on a full-format sensor).
Most of the time there were no problems. I preferred taking photos of things in their natural environment, with little setup or arrangement, and using natural light instead of flash emphasized that. It was not easy getting shots during bad weather on the forest floor, where it gets dark, but usually a tripod would take care of that: plenty of shots were successful at 1/8 and 1/15 second exposures. (There's an example here]. The main problem is wind: you need about 1/200 second to freeze a flower when it's blowing around. With your camera and lens combination, though, the lens is one stop brighter and you can easily push two more stops to ISO 1600, so what I was shooting at 1/30 second you can shoot at 1/200 second (with a bit less depth of field).
Another consideration for me is that this macro was so versatile it became my walk-around lens. It would have been unwieldy to carry along a macro flash with it. I could take trips into the woods lasting weeks and photograph everything I saw, from the very small to the very large, with this lens (and two other small, lightweight primes at 28 and 55 mm), and not feel burdened by the weight and without needing any battery replacements. The equivalent combination for the 60D, in terms of covering field of view, would range from about 18 mm through a 60 mm macro. If you supplement the 100 mm macro even with the kit 18-55 mm lens, you have that range covered plus a light telephoto: there's an awful lot you can do with that setup.