You can roll how you want to, as long as you are willing to live with the consequences.
One of the things I love about Brazilian jiu-jitsu (BJJ) is the diversity of styles and personalities I have found on the mats over the years. Even when all of the people in the room have the same teacher, each practitioner, over time, develops his or her own unique style of jiu-jitsu. There is no denying the influence of a teacher, training group, or a lineage and all of these certainly manifest in the practitioner’s development, but attitude, body type, and personal philosophy are as much or more evident in the way a particular BJJ practitioner rolls (spars) over time.
No two BJJ practitioners roll exactly alike. One person may tend to flow and move, always looking for advantageous angles from which to apply techniques against little or no possible resistance. Another practitioner may prefer to apply pressure and leverage to slow down his or her opponent, trapping him or her into a position from which there is no room to escape before applying a submission. Likewise, people like to explore the limits of their own flexibility, explosive power, speed, or cunning. None of these approaches to BJJ is inherently better than another, but each has its advantages and each has its weaknesses.
The goal is to train consistently and intelligently enough within your preferred method, so as to be able to track and measure progress, to amplify strengths and shore up weaknesses. This means making choices. If the way you roll is totally random and chaotic, your results are likely to be as well. If, instead, you are focused and have a particular goal or set of goals, a specific skill you want to work on or an approach to the art that you want to explore, focus on that against all of your partners, win, lose, or draw. By being disciplined in this way, your personal style will develop such that you can be yourself regardless of the challenge you face.
“As in life, so too it is in budo. As in budo, so too it is in life.”
-Robert Van Valkenburgh is co-founder of Taikyoku Mind & Body and Kogen Dojo where he teaches Taikyoku Budo and Brazilian Jiu-Jitsu