I just upgraded my MTB with some Forte City Slicks are they are 1.5 tires. The tubes I have from my old tires are 1.75-2.00. Would having tubes that are that much bigger be an issue on them? I rode with them today without any problems so far! They might be a little wobbly, but nothing really that off. Should I really consider a smaller tube or will leaving them as is not going to cause any damage?
Not generally a problem, though if the tubes have to be folded up and squeezed to fit, there's at least a theoretical chance of rubbing or pinching a hole in them. FWIW, I buy only two sizes of tube: 26x1.5 for our mountain bikes and 700x25 for the road bikes. They go into everything from 26x1.4 to 2.3, and 700x23 to 41. No problems ever.
What's "wobbly," though? If you can feel the tires squirming around on the rims, check your pressures. Don't run the skinny road tires at the same low pressures you ran the big knobbies.
....Should I really consider a smaller tube or will leaving them as is not going to cause any damage?
I would say to get some tubes that cover the tire size. I have ran fat tubes in narrow tires and seen cracks where the tube was folded over inside the tire (as well as felt the lumps while rolling!). And at the opposite end--a narrow tube used in a fat tire will tend to leak air and develop holes, because it's stretched farther than it's really intended to.
My guess is that they probably make tubes in different widths for a reason.
I have bought some Bell tubes at Walmart and some Schwinn tubes at Target. Both say on the package that they cover my tire size range, 26x2.125. However, both brands of tubes are very skinny, and have to expand about double to fill that tire up. I would say if the tube is not obviously larger than the tire (if there's not any wrinkes in it when in place), it's good to go.