You can blow any door , locked or not locked for sake of saving time ( and what I usually do ) Also if everyone get shotguns, you got enough ammo of breaching many many doors.
Theres another very useful hint I want to share. Flash works great everywhere, in that case helmet guy can enter as soon as the flash still flying in midair. Thats how British SAS works. They got antiflash lenses on gas masks, just like welding masks. But its important that you got timing right. have to work a lot on timing with teammates to make it perfect.
Me and YoursTruly almost got that perfectly
Breacher counting and blowing door meanwhile flash going in side as soon as door starts moving ( means throw need to be made before door been shot ) Then point man goes in with the flash and not waiting til it blew up behind corner