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 $123.82   12.316%   $139.0697   $139.07   $139.05   $139.10  
  Purchase #2 $60.72   6.498%   $64.6656   $64.67   $64.65   $64.70  
  Purchase #3 $123.68   7.347%   $132.7668   $132.77   $132.75   $132.80  
  Purchase #4 $32.29   3.610%   $33.4557   $33.46   $33.45   $33.50  
  Purchase #5 $219.47   12.704%   $247.3515   $247.35   $247.35   $247.40  
  Purchase #6 $155.56   12.821%   $175.5043   $175.50   $175.50   $175.50  
  Purchase #7 $155.70   4.634%   $162.9151   $162.92   $162.90   $162.90  
  7 purchase totals = $871.24       $955.7287   $955.74   $955.65   $955.90