One of the benefits of doing a receiving line is that you get to formally say "hello" to all of your guests before your reception starts. This can eliminate the need to go around to all of your tables during the reception-- although proper etiquette would tell you to still do both.
One downside of doing a receiving line is that it eats up a lot of time- especially if you have a large wedding. Some guests will want to talk to you for several minutes, holding up the line. If you are pressed for time you need to take this into consideration.
Here's when I suggest you don't do a receiving line: Your ceremony and reception are at the same venue, and you only have the venue for a total of 5 or 6 hours. You are pressed for time and your only option for bridal portraits is between your ceremony and reception...and you actually want to be at your cocktail hour.
Also, if you are having a super small wedding there really isn't a need for a receiving line. You can easily greet all of your guests during your reception.
Receiving lines are considered a traditional part of a wedding, however with so many couples now opting for non traditional weddings, receiving lines seem to be a thing of the past. If you decide to for-go a receiving line, you need to carve out time to say hello to each of your guests during cocktail hour or your reception. Nothing makes a wedding guest more upset than not getting to talk to the newlyweds.