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 $36.81   8.587%   $39.9709   $39.97   $39.95   $40.00  
  Purchase #2 $125.72   11.471%   $140.1413   $140.14   $140.15   $140.10  
  Purchase #3 $20.51   8.408%   $22.2345   $22.23   $22.25   $22.20  
  Purchase #4 $129.74   5.530%   $136.9146   $136.91   $136.90   $136.90  
  Purchase #5 $103.52   12.512%   $116.4724   $116.47   $116.45   $116.50  
  Purchase #6 $177.49   11.123%   $197.2322   $197.23   $197.25   $197.20  
  Purchase #7 $111.91   11.762%   $125.0729   $125.07   $125.05   $125.10  
  7 purchase totals = $705.70       $778.0388   $778.02   $778.00   $778.00