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 $21.47   4.808%   $22.5023   $22.50   $22.50   $22.50  
  Purchase #2 $9.46   13.565%   $10.7432   $10.74   $10.75   $10.70  
  Purchase #3 $99.79   5.317%   $105.0958   $105.10   $105.10   $105.10  
  Purchase #4 $31.48   4.681%   $32.9536   $32.95   $32.95   $33.00  
  Purchase #5 $150.41   5.680%   $158.9533   $158.95   $158.95   $159.00  
  Purchase #6 $186.28   4.514%   $194.6887   $194.69   $194.70   $194.70  
  Purchase #7 $86.25   5.503%   $90.9963   $91.00   $91.00   $91.00  
  7 purchase totals = $585.14       $615.9332   $615.93   $615.95   $616.00