Ideally, every roll would start with both opponents standing. Potential collisions with other people rolling nearby make this impractical on a crowded mat. Additionally, takedowns often result in wear and tear on the body. Most gym rolls have an implicit agreement that both opponents will avoid standing at the same time.
The naÃ¯ve compromise is to start rolls with both opponents on their knees. The assumption is that this allows both participants to fight for top position while avoiding the potential injuries associated with stand up grappling.
The problem is that both-opponents-on-their-knees is a contrived situation that doesn’t occur in competition. It doesn’t even occur during a friendly gym roll unless you purposely start there. One opponent simply needs to stand to gain an immediate positional advantage over their kneeling opponent.
Even worse is that the both-opponents-on-their-knees situation keeps the bad aspects of jiu-jitsu standup (both opponents neutralising each other in a tight clinch) and loses the good ones (mobility to gain favourable angles).
A better way is to start in a guard position, the most common being closed or seated. It is a more effective use of training time to start in the guard, than to struggle on the knees for a minute or so before ending up in the guard anyway. In competition, most time is spent in the guard so your training time should reflect that.
An alternative approach (especially for the more experienced practitioner) is to start in an inferior position. This allows you to practise your escapes directly, rather than allowing your opponent to pass and establish dominant position before beginning an escape. Unless you are specifically working on the timing of your escapes, allowing your opponent to establish dominant position so you can practise your escapes just builds bad habits.