I am not a SBT breeder or show person however, have quite a bit of showing experience with bull breeds.
I am going to answer yes and no to your question as the focus on the bite of a dog in the show ring will be individual for judges. It is possible to do very well with an undershot bite and also possible to do very poorly. Some judges will overlook shocking mouth faults and it's all that others will consider.
If the standard calls for a scissor bite then that is the ideal and I think you will find in your standard wording to the effect that any departure or fault should be considered proportionally to its extent and impact on the dogs health.
My opinion only, in a SBT firstly I would be looking to see if the canines are correctly aligned- this generally indicates that the jaw is correctly aligned and there is good balance between the upper jaw and underjaw. If the incisors are level or just undershot (sometimes referred to as reverse scissor) this is a fault however, I would not consider it as serious as a gap between the upper and lower incisors (or a gap the opposite way - overshot). The greater the gap, the greater the difficulty the Dog will have from a health perspective.
So a few tips:
1. Check that the canines are correct
2. Check if there is a gap in the undershot bite between the upper and lower incisors
3. The greater the departure from the ideal, the greater the chance a judge may view it unfavorably
Personally, I would be reluctant to show a dog with a mouth fault, even minor unless it was of amazing quality otherwise, and you would always need to be prepared for and accept it may be viewed negatively and go against you.
I think every one who shows would agree that literally anything can happen, so I don't want to curb your enthusiasm. All I am saying is that you need to understand your dogs faults and understand and be aware of what can happen. Teeth are an obvious part of the dog so they usually will be scrutinised.