Most times there are very few problems if you don't prewash. I don't like the feel or smell of the unwashed yardage and sometimes they give me issues. I've been saved before when pieces that seemed fine before washing turned out to be really bad greige goods (the base fabric) even though they were expensive, quality brands were flimsy, raveling and unusable after washing. I've also had iffy fabric come out better after a prewash but somehow that doesn't happen often.
I prefer to wash my yardage along with my regular like-colored laundry, I keep a pack of Color Catchers on hand for use. I wash on warm with an unscented detergent. We have good water here, I don't have any problems with minerals and don't use a water or fabric softener. No dryer sheets either on my yardage. For the last several months I've been using the wool dryer balls and feel that they are making a difference.
I hate ironing but that might because I have high standards
and so when I have lots of yardage in the dryer, I watch it carefully and pull out the yardage just before being totally dry and give it a good pressing. I don't use starch, but my finished ironed fabric is very smooth and people are often surprised if I go with fabric to a group session and they find it's been prewashed.
I had a quilt ruined once by an unstable red/maroon fabric. All you need is one project to go very bad to want to never have that happen again. Turned the sophisticated maroon/grey/blue/silver with bright bits of sparkling white into not so sparkling baby pink. It was a reputable brand from a reputable shop and I also got a blue in the same line that had no problems. I've shown the picture below before, the "orange" fabric was the actual fabric that ruined my quilt. The still maroon fabric I found in a thrift store, it is the same design as the blue I used in the quilt. The maroon has been washed at least twice, the orange has been washed at least 6 times and still runs like an Olympian sprinter!
I find reds and dark blues to be the biggest culprits. I've had bright red flannel "crock" that is, dye transfer from one stored (non-washed yet) piece to another as well just by being in contact with each other. Now if I'm unsure and haven't tested it yet, I wrap the piece in acid free tissue paper until I do test it.
One of the easiest ways to test for dye stability is simply hot water in a coffee cup. Put a swatch of fabric (4-6") in a cup with a white interior and pour some boiling water on it, or if you have a Kuerig type system, just take out the pod of coffee and let the hot water run in. Let sit for a 5 minutes or so, you can poke at it or swirl it around (be careful and use a something other than your finger...). Lift up the swatch and squeeze out the water and see if what remains looks white (good!) or not (bad bad bad). If there is just a tiny amount of color, take your chances and either don't prewash or just throw it in the laundry and don't think about it.
But if there is a lot of color, one prewash may not be enough so test it again. Typically I keep an original piece of fabric to compare against fabric if I'm concerned, helps me judge whether to keep going with that particularly piece of fabric.
I recently bought a bag of home-dyes at the thrift store, I could tell they had been treated with something but it turns out that the dye was never properly fixed, just dyed and then starched apparently. When I tested my first piece, the coffee cup promptly turned a color suitable for egg dying. Sometimes you have just one unstable color, but no... none of these were properly treated and the purple and emerald green dyes seemed especially cast off, the reds weren't near the issue as the darks. Since I don't know what dyes/inks/processes they were made with other than they seem to be mostly artificial dyes, I've been dealing with them for the last several days. I start with a machine pre-wash. I then dip a couple of pieces at time into boiling water for a 8-10 minute "dunk". Then I rinse clean. Then after all the pieces are washed, I boil them for 15 minutes in a bath of water/salt/vinegar. Again, rinse until clean. Then, they go in small batches into the microwave for a 8 minute boil followed by a bit of a cool down and another rinse until clean. Finally after all that dipping and dunking and boiling and rinsing, they go back into the washer for one last load with detergent and drying. The color catchers are coming out barely turned, which I don't consider bad from a load of laundry. But it's a lot of work and it would have been a lot easier just to trash this bag of fabric. Since my car was in the shop and I was stuck home for a few days it did give me something to do but more than once I was questioning my sanity.