A holster that exposes the trigger guard and trigger of a double-action revolver is safe enough so long as the gun fits the holster, and one keeps their finger off the trigger while drawing until the muzzle is safely pointed down range.
The same can be said concerning single-action revolvers so long as the hammer isn't cocked until the muzzle is pointed in a safe direction.
However certain pistols, the Glock and cocked & locked 1911's in particular, should always have the trigger guard covered, and again keep the finger off of the trigger until the muzzle is pointed in a safe direction.