We make PS for at least one restaurant per day. Lunch is a good one, as the PS are easier to get, and the prices are a bit less expensive then. PS during lunch time, except for perhaps the character meals or San Angel Inn, Sci Fi Dine In, 50's Prime Time Cafe and the Brown Derby could be optional, if you are going during a slower time of the year. If you are going at a more busy time of year, it would definitely behoove you to make PS for any sitdown meal.
For breakfasts, any character meal will require a PS. See above for lunch, and at Dinner, most restaurants in the World Showcase would need one, as well as any character meal, Coral Reef, Cinderella's Royal Table, any restaurant at Disney MGM, as many participate in the Fantasmic dinner package (which definitely requires PS), California Grille, 'Ohana, Narcoossee's, Flying Fish, Cape May Cafe, Yachtsman, Blue Zoo, Palio, Shula's, Fulton's, Portobello Yacht Club, Citricos, Artist's Point and Whispering Canyon.
You may call ahead to 407 WDW DINE and make your PS in advance... they start taking PS 90 days ahead of time, and for character meals, California Grill, Artist Point, and 'Ohana it is recommended to get them ASAP. Or you can wait until you get to WDW and make them when you check in if you are staying onsite, or in your room (just push the dining button), or at the Guest Relations at any of the parks. Disney MGM Studios has a separate place to make your PS... it is called Hollywood Junction and is located on the corner of Hollywood and Sunset, next to the tip board!