Yeah, your right that is how you construct a seventh chord. You'll usually find the 5th dropped in a V7-I cadence. The 5th in a seventh chord (particularly dom7th) is the one tone you can leave out, and still have the chord function the same way. The two important tones in a seventh chord are the 3rd and the 7th, while the root which tells you how it will function. The 5th doesn't really do anything for the sound of the chord either, unless it's altered. It's always better to use fewer notes in a chord to achieve the sound you want. The more notes, the more cloudy it sounds. You can still play the 5th, but you should ask yourself do you really need to?