Sometimes trainers will try get the standown over ridden so dont scratch until the last minute also some trainers have the second reserve and the dog they are scratching has a very good box like box 1,2or 3 so will not scratch until very late hoping for another to scratch so the reserve gets the good draw
I dont think many clubs pass on scratching until they have to I could be wrong
I know that most clubs let the reserve dog know as asp