You always want your ISO as to the highest number possible. To prevent the motion blur as already stated you want a fast shutter speed.
I will respectfully say, absolutely not.
Proper exposure is based on a triangle. The three components are aperture ("f stop"), shutter speed and ISO (sensitivity).
By raising your ISO unnecessarily high, you're introducing noise and degrading the image, especially on a previous gen sensor like on the D90.
The first basic steps; with the 18-200, try to shoot as wide / close as you can (IE, get closer to the action). Why? The 18-200 is a variable aperture lens; by 100mm you're already at a slow f/5.6. If you were to shoot at say, 50mm which will open up to f/3.5, you're letting in three times the amount of light, allowing you to set your shutter 3 times faster.
For example, the following settings provide the same exact exposure;
f/2.8 - 1/250
f/4.0 - 1/125
f/5.6 - 1/60
These would be approximate settings for shooting a night parade. At 2.8 you can set your shutter to 1/250 which will freeze even fast action. As you can't shoot at this aperture, you're at f/4 as a minimum; you are letting in half the amount of light, so now you have to double your shutter speed to maintain a proper exposure, so now you're at 1/125, which is still suitable for slow moving parade's (especially if you pan your shot with the movement of the float). At 5.6, you are now letting in 4 times less light that at 2.8 and now have to quadruple your shutter speed, all the way down to a slow 1/60, which will lead to image blur for shooting moving objects.
Once you get close enough to the action, as Gianni's Papa said, I would recommend setting your camera to shutter priority mode at 1/125 as a minimum, the camera will keep your aperture set as wide open as your lens allows. Alternately you can shoot full manual and try that route. I would start with an ISO setting of 800. Take a few shots and check them. Underexposed? Increase your ISO. Take a few more shots, check it again. If for some reason it's actually bright enough that the body is pushing the aperture up to say, f/8, dial your ISO back a notch (from 800 I believe 640 is the next setting). The long and short of it, you want to keep your ISO as low as possible while still maintaining a proper exposure. Do some test shots BEFORE the parade begins, but not so early that you've lost a bunch of light from the sun setting and you now have to compensate for that.
There are a few exceptions to the above. Lets say you got your hands on a nice fast 50mm f/1.4. At f/1.4, you are letting in sixteen times the amount of light compared to f/5.6. So in reference to the above examples, you would be able to set your shutter at 1/1000 to maintain the same exposure. Shooting wide open like that though can lead to unwanted effects, specifically a VERY shallow depth of field. What that means is that the subject will be sharp on that focal plane, but will then soften at different depths. (IE, Belle's face will be sharp, but the beast that is standing 3' behind her will be blurry, as will the rest of the float). Shooting wide open is idea for portraits, not so much for parades.
HTH's!
