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 $18.93   4.419%   $19.7665   $19.77   $19.75   $19.80  
  Purchase #2 $160.64   4.220%   $167.419   $167.42   $167.40   $167.40  
  Purchase #3 $92.56   9.874%   $101.6994   $101.70   $101.70   $101.70  
  Purchase #4 $5.66   5.666%   $5.9807   $5.98   $6.00   $6.00  
  Purchase #5 $231.67   9.507%   $253.6949   $253.69   $253.70   $253.70  
  Purchase #6 $174.87   5.745%   $184.9163   $184.92   $184.90   $184.90  
  Purchase #7 $240.62   6.690%   $256.7175   $256.72   $256.70   $256.70  
  7 purchase totals = $924.95       $990.1943   $990.20   $990.15   $990.20