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 $12.29   5.643%   $12.9835   $12.98   $13.00   $13.00  
  Purchase #2 $12.38   4.642%   $12.9547   $12.95   $12.95   $13.00  
  Purchase #3 $122.46   13.735%   $139.2799   $139.28   $139.30   $139.30  
  Purchase #4 $229.57   10.496%   $253.6657   $253.67   $253.65   $253.70  
  Purchase #5 $18.42   11.112%   $20.4668   $20.47   $20.45   $20.50  
  Purchase #6 $202.29   11.518%   $225.5898   $225.59   $225.60   $225.60  
  Purchase #7 $8.32   6.422%   $8.8543   $8.85   $8.85   $8.90  
  7 purchase totals = $605.73       $673.7947   $673.79   $673.80   $674.00