It depends on the scheduler being used. In most cases, it will not matter.

With Perfect-N Type schedules, you will not get empty slots (Byes) in the schedule, unless you select one of the upgrade options to add a Bye to achieve a better quality schedule.

Normal, Modified and Phase Shifted Lane Rotation schedules are the same way. Though, with Clustered Lane Rotation schedules, you will end up with one or more Byes added to make the number of racers divisible by the number of lanes. If possible, these Byes will be spread out, so you don't have more than one Bye per heat.

With Dynamic scheduling, you will end up with one or more Byes added to make the number of racers divisible by the number of lanes. If possible, these Byes will be spread out, so you don't have more than one Bye per heat.

I you are using some other scheduling method, then it is up to that scheduling method on how it deals with that situation.