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 $18.90   10.412%   $20.8679   $20.87   $20.85   $20.90  
  Purchase #2 $190.56   13.866%   $216.983   $216.98   $217.00   $217.00  
  Purchase #3 $78.84   3.129%   $81.3069   $81.31   $81.30   $81.30  
  Purchase #4 $79.47   10.277%   $87.6371   $87.64   $87.65   $87.60  
  Purchase #5 $12.76   7.858%   $13.7627   $13.76   $13.75   $13.80  
  Purchase #6 $144.39   9.290%   $157.8038   $157.80   $157.80   $157.80  
  Purchase #7 $212.61   6.155%   $225.6961   $225.70   $225.70   $225.70  
  7 purchase totals = $737.53       $804.0575   $804.06   $804.05   $804.10