Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $129.80   5.733%   $137.2414   $137.24   $137.25   $137.20  
  Purchase #2 $229.35   6.178%   $243.5192   $243.52   $243.50   $243.50  
  Purchase #3 $179.52   6.430%   $191.0631   $191.06   $191.05   $191.10  
  Purchase #4 $134.77   10.703%   $149.1944   $149.19   $149.20   $149.20  
  Purchase #5 $8.58   9.236%   $9.3724   $9.37   $9.35   $9.40  
  Purchase #6 $9.59   3.108%   $9.8881   $9.89   $9.90   $9.90  
  Purchase #7 $172.60   4.684%   $180.6846   $180.68   $180.70   $180.70  
  7 purchase totals = $864.21       $920.9632   $920.95   $920.95   $921.00