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 $16.93   8.136%   $18.3074   $18.31   $18.30   $18.30  
  Purchase #2 $129.73   7.523%   $139.4896   $139.49   $139.50   $139.50  
  Purchase #3 $83.31   8.834%   $90.6696   $90.67   $90.65   $90.70  
  Purchase #4 $152.31   4.703%   $159.4731   $159.47   $159.45   $159.50  
  Purchase #5 $125.56   6.170%   $133.3071   $133.31   $133.30   $133.30  
  Purchase #6 $12.96   6.318%   $13.7788   $13.78   $13.80   $13.80  
  Purchase #7 $91.73   11.274%   $102.0716   $102.07   $102.05   $102.10  
  7 purchase totals = $612.53       $657.0972   $657.10   $657.05   $657.20