The "First look" is an idea that is pretty new to wedding photography. There are arguments for and against it and honestly I think either one is fine, it is mainly just a personal preference but there are a few factors to consider in choosing whether or not to.
What is the "first look?" It is a short session before the wedding with the couple and the bride and groom. It can be a surprise moment for example: the bride might be hidden or her back might be turned from the groom and then the interaction/joy of seeing each other is caught on camera or it can just be a simple portrait session or both.
Why would you want the portrait session first? Mainly to save time after the ceremony AKA more time to spend at the cocktail hour! It also makes some couples a little more relaxed during the ceremony and gives them a short quite time away from all the hub bub of the wedding. Some couples even do a short shoot before and after.
So what are the factors you would want to think about? One thing to think about when considering a "first look" is the timing of your wedding. Will it still be daylight after your ceremony? If not you may want to grab a few shots before. Another thing to think about in terms of lighting and time is your location and when "magic hour" (right before sunset) happens. If magic hour is after your ceremony and there is not a lot of shade I would recommend doing the portraits after the ceremony.
Why is magic hour so great? It is when all the soft whimsical light happens and is way more flattering then the harsh afternoon sunlight and it creates a beautiful background.
Why do some couples wait till after the ceremony? One reason some couples may want to wait is because they want that anticipation of seeing the bride or groom walk down the isle for the first time or it might be a custom that culturally you don't want to change.
Basically, though it comes down to whether you want more time for cocktails/schmoozing, lighting and if it is important to you as a couple to see each other (or not) before the ceremony. After all it is your wedding and in the end you want to do what makes you happy.
Tip: Not sure on the best lighting & timing for your wedding? Ask your photographer and they will be happy to give you their advice.