Astrostu... I know your name as someone that has been around here likely before me. But honestly... if you need to ask if you need an umbrella at a wedding... you are clueless as to what is needed to pull off a wedding properly. Please, no offence, but weddings are a heck of a lot harder to shoot than star trails or fireworks.
The good news is you have some time to prep and get ready for an event that is VERY VERY VERY important that you do right.
1. Make sure you have
MASTERED the basics of photography. You can talk ISO, aperture, shutter speed, chew gum and walk all at the same time without forgetting to breathe.
2. Take the time to mentor with a KNOWN GOOD EXPERIENCED WEDDING PHOTOGRAPHER. Mentoring is the BEST and FASTEST way to learn. This way you can play "mr pro wedding photographer" and no one gets hurt, becuase someone is covering your back and at the same time teaching you how to do it right.
3. Take the time to
MASTER your camera and lighting equipment. If you have to think about how to change WB from sunlight to tungsten to shade... it's time to get so good that you can do this without thought. As an example, I can now adjust WB settings, ISO, settings, aperture settings and shutter settings with my eyes shut and still know where I am and what I have changed my settings to. This may sound extreme or stupid here... but when you are in a dark church, you don't have time to chimp settings, change them and chimp again. Change them now, and get that shot!
4. If your equipment is not capable if capturing excellent quality shots in difficult and very low light situations, rent it, learn to use it. If your camera is not clean at ISO 1600, it is the wrong camera. If you do not have lenses that range from 30mm to 200mm and all are F/2.8 or faster, you will miss so many shots, its not even funny.
5. If your post processing skills cannot result in pictures that cannot bring smiles or even tears of joy, now is the time to learn, but dedicate yourself... learn and see how the other wedding pros are doing it, and do something that you can be proud of and something that puts a smile on the bride's face now and 20 years later as she is looking at your photos.
Off camera lighting is very important, at least in the style that I have come to use. There are a few that are ambient only, and they get away with it... but either their equipment is INCREDIBLE or they simply choose not to shoot in challenging situations (ie: miss tons of shots).
To answer your question... nah, I can get away without using an umbrella or softbox, but there are times it is an advantage... the getting ready in the homes shots or after church couples or small group shots, the cake shot, the table settings (center piece) shots come to mind rather fast. Other than that, I don't use or need an umbrella at all. I do use diffusers and bounce cards a LOT, though. It is easy to walk around with a tripod with a speedlight and a bounce card on it... way harder if that lightstand has an umbrella/softbox.