Well, it's all good when you do it right. There are plenty of online resources to help you figure out what size you should be looking at when its time to upgrade your tires. The real question is why?
Part of the answer is that bigger wheels equals more bling. Whether stealthy black or flashy chrome, bigger wheels just look better. When it comes to the tires that you'll be using with the new rims, size matters. Your ride was designed to use a specific size of tire and the important part of that is the circumference, or the distance around the outside of the tire tread.
For a specific circumference, the tire will complete a certain number of revolutions for every mile traveled. This number is used by the speedometer to display your current speed. A tire with a smaller circumference will complete more turns per mile and the speedometer will read high as a result. A tire with a larger circumference will complete fewer revolutions per mile, and the speedo will read low.
The other aspect of tire size is the width. Wider tires can improve handling, but the size of the wheel needs to be right. Any size of tire can be used with a range of wheel widths. The tire manufacturer's specs will state minimum, maximum and "measured" wheel widths. The measured width is the one they used to generate the published specs for that tire size.
When looking to get bigger wheels, you want to keep the overall tire circumference about the same as before. To do this, you need to use a lower profile tire. For example, the factory tire for a 2009 Honda Accord Sedan EX V6 is a 225/50R17. If you were upsizing to an 18-inch wheel, the recommended tire size would be 235/45R18. For a 19-inch rim, it would be 245/40R19 and if heading for the 20" barrier, you'd be looking for a 245/30R20 size.
Aside from budget, one thing may stop you from making the big jump and that is the tire width. In order to maintain the correct circumference, the tire has got wider... in the most extreme case by 20 mm, or just shy of an inch. This could cause rubbing problems when driving, so check with other people on the forums to see if this has been a problem before.
Looking at the recommended tire sizes, you'll see that the substitution isn't exact, but it's close enough for most purposes. The overall diameter on the 20-inch tire is 25.8-inches, while the 19-inch is 26.7, the 18-inch is 26.4-inches and the original size is 25.9-inches. So, each size will have some impact on speedometer accuracy, but it's just a couple of percent.
Understand that bigger wheels add a great look, but can bring performance penalties along with them. Often heavier than the originals, large wheels require more power to get up to speed and can reduce both handling and ride quality. Increased unsprung weight is responsible for the handling impact, while the stiffer sidewalls of ultralow profile tires affect the ride.
If its all a bit too confusing for you, check with the Discount Tire people here on Modded, a tire professional in your area, or post specific questions about suitable tire and wheel brands, sizes and fitments to the appropriate forum section here.