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 $25.59   13.571%   $29.0628   $29.06   $29.05   $29.10  
  Purchase #2 $111.92   9.663%   $122.7348   $122.73   $122.75   $122.70  
  Purchase #3 $202.87   3.494%   $209.9583   $209.96   $209.95   $210.00  
  Purchase #4 $82.55   4.388%   $86.1723   $86.17   $86.15   $86.20  
  Purchase #5 $221.75   8.855%   $241.386   $241.39   $241.40   $241.40  
  Purchase #6 $59.99   10.574%   $66.3333   $66.33   $66.35   $66.30  
  Purchase #7 $199.59   12.490%   $224.5188   $224.52   $224.50   $224.50  
  7 purchase totals = $904.26       $980.1663   $980.16   $980.15   $980.20