Why you should consider using a gridDrawing things isn't always easy. Even with practice, somethings are still tricky. Grids are extremely useful tool that can help reduce mistakes and make it easier to get things right. Don't worry about the grid itself messing up your image; it's easily removed if you do your inking on a computer.
Most art programs have a feature that displays a grid behind or on top of the current image. This overlay is not actually part of the image and won't be saved, so again, don't worry about it getting in the way.
Grids come in handy for two things: proportions and positioning. Many, if not most, of the tutorials on this site suggest measurements when working on details of the drawing. If you use a grid, you can make measurements a snap. For example, take a look at the tutorial for the Creator's Star. It suggests that you should make a box 7/8ths as wide as it is tall. Normally you'd need a ruler or something to measure the height, divide the amount by 8, multiply by 7 (to get seven eighths) and then measure your line. Using a grid, it's much much easier: make the box 7 squares wide and 8 squares tall. Additionally, the tutorial calls for you to divide the sides into fourths. No measuring is needed here either: one-fourth of the height is simply 2 squares on the grid!
As mentioned above, grids can also be very useful when you're positioning something. Placing an object in the center of an area is simply a matter of counting squares; leave an equal number on each side and you've centered your object!
Even better, some programs like Flash offer a 'snap to grid' feature. This feature allows you to move the mouse more freely but have tighter precision. Every line drawn will 'snap' to the nearest corner in the grid. This is invaluable when it comes to doing some of the more straight shapes, as the lines will always be crisp and straight. This can easily get in the way however, so be sure to know how to disable it when you don't need it anymore.