Depends on what you mean few mm. In my experiment it is important .I hold the slingshot sideways so if one of my band is shorter, then as I experienced it will pull the pouch faster and the ball will go either up or down of the target.not crutial at all.
Slingshot is a rough 'projectile' tool no matter what high-tech are used in making it.
The accuracy depends mainly on the shooter. A few mm.doesn't matter at all.
I try to put the bands on the pouch even both sides .Then I attache the other end to the fork and I let the pouch hang free while the slingshot is in horizontal position this way It hangs freely and I realy can see if 1 band is sorter then the other.
Also slingshot is basically very simple 1 Y shaped wood ,2 bands and a pouch but as I experianced there are manny things that needs to be look out for.esspecialy with tapered bands. If one shoots instinctively and not an experianced shooter might not see the different but I use the fork to aim and this way even I can see if my setup doesn't shoot the way it suppose to.(it happends many times)
What do you guys think about this ???