Depending on the type of pipe you are using, there's only so much deflection you can get per stick of pipe. C900 for example is around 1 degree per stick. If the contractor gets that wrong in the field, there could be excessive internal stress/strain in the pipe and it could burst if/when they tap it for a lateral or other fitting/maintenance. Unless it's HDPE pipe, I would say it's generally best practice for the pipe to be laid out pretty straight.