At 18.87 I would say they are close to optimum, better to be a little smaller than larger but you work with what you got. Chances are 98% of the glass with a 19mm ID will work fine as this is within what we normally see as drift. This is beyond the manufacturers spec so there is no way we can guarantee every piece to be above that but as most who have dealt with us know we are pretty good on the support end and will always take care of the customer.
Another thing to understand when we are talking about tolerances, flame polishing will reduce the minimal opening. What happens is the glass melts and curls over like a mushroom cap. We cold work all of our glass, this makes things a little rougher in the work end as you have to be able to cut and finish without chips that most polish out with flame but we have concluded that maintaining the manufactured annealing and tolerance is worth the tradeoff for the extra care in the cutting and finishing.