First, twin flames are 2 halves of the same soul incarnated and housed in 2 separate human bodies: one half male energy and the other half female energy.
Twins have been known to dream of their respective twin before meeting in the physical 3-D, during their journey and in 5-D as well along with post-union experience. Is it common? Sure, but it's not required or necessary. It's not abnormal to experience a dream then forget it prior to waking. You may be dreaming of them and not even know it. And they, you.
The focus of the twin flame is typically their divine mission, so it makes sense for twins, whether in union or not, to dream of just about anything. Dreaming of someone also can in some cases denote chronic or obsessive thinking and since twins do not exclusively obsess, then its reasonable to assume that dreaming of one another may not be commonplace for all twins.
YOUR journey and union with your twin is unique to your partnership. This includes dreaming — or lack thereof.
When twins come into union, lucid dreaming and astral projection accompany ascension for many twins. The most common dream I hear of from clients is dreaming of their divine mission or messages from their twin. Your twin resides with you, so while any twins are able to connect in 5-D or on the astral dream plane, you and your twin may not feel the need to, as you can communicate right here and now, because you are both in one another's presence physically as well as spiritually.
Dreams can bring messages about your path together, your journey, your reunion or union, and your mission together as Twin Flames.