The are pros and cons for a 7/10mins qualifiers.
It may also depends on the number of participants. For example, if it a club race with fewer number of participants, and if time permits, the pros may come in having the pit-men having learnt about good pitting skills. Otherwise, just stick to 5mins and have all the heats and mains completed.
In a bigger scale race, we can stick to 5mins due to the large participants list. We may not be able to complete the heats in time etc etc.
My take is whatever the decision, I just run it and hope for the best.
In terms of cost, you can say that it may be higher for longer timed heats. Tyre wear factor, more fuel used and yeah prob the 3.5min qualifying engines.
In the end, i still say r/c racing is about meeting up with frens of the same interest and have great fun.