The short answer to this question is a resounding YES! No sooner than 1 week after installation and no later than 1 month after installation, your structure should be retested and a follow-up report comparing the before and after analysis results should be presented with a recommendation. Otherwise, how will you know if the RADON Mitigation system is actually mitigating the RADON in your home...? Finally, followup testing should be done using Granular Activated Carbon canister(s) [GAC]. GAC will rarely to never give you a false positive test.
This retest should be included in the cost of your mitigation system installation, as should the followup report.
If the results of the followup show that there is little or no difference between the test results, your RADON Mitigation Professional should make any and all adjustments to make your mitigation system effective. AT NO ADDITIONAL COST TO YOU! If adjustments are made, then another retest should be conducted to prove effectiveness...again, using GAC, AT NO ADDITIONAL COST TO YOU!