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 $128.54   8.116%   $138.9723   $138.97   $138.95   $139.00  
  Purchase #2 $211.88   11.823%   $236.9306   $236.93   $236.95   $236.90  
  Purchase #3 $148.96   7.786%   $160.558   $160.56   $160.55   $160.60  
  Purchase #4 $37.73   12.501%   $42.4466   $42.45   $42.45   $42.40  
  Purchase #5 $39.96   10.105%   $43.998   $44.00   $44.00   $44.00  
  Purchase #6 $48.53   10.512%   $53.6315   $53.63   $53.65   $53.60  
  Purchase #7 $85.71   6.373%   $91.1723   $91.17   $91.15   $91.20  
  7 purchase totals = $701.31       $767.7093   $767.71   $767.70   $767.70