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 $84.65   8.368%   $91.7335   $91.73   $91.75   $91.70  
  Purchase #2 $130.97   5.861%   $138.6462   $138.65   $138.65   $138.60  
  Purchase #3 $222.58   4.502%   $232.6006   $232.60   $232.60   $232.60  
  Purchase #4 $27.69   10.819%   $30.6858   $30.69   $30.70   $30.70  
  Purchase #5 $112.80   12.650%   $127.0692   $127.07   $127.05   $127.10  
  Purchase #6 $175.78   3.816%   $182.4878   $182.49   $182.50   $182.50  
  Purchase #7 $155.47   3.522%   $160.9457   $160.95   $160.95   $160.90  
  7 purchase totals = $909.94       $964.1688   $964.18   $964.20   $964.10