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 $19.80   9.254%   $21.6323   $21.63   $21.65   $21.60  
  Purchase #2 $126.44   10.188%   $139.3217   $139.32   $139.30   $139.30  
  Purchase #3 $153.62   13.211%   $173.9147   $173.91   $173.90   $173.90  
  Purchase #4 $166.29   4.371%   $173.5585   $173.56   $173.55   $173.60  
  Purchase #5 $35.55   4.682%   $37.2145   $37.21   $37.20   $37.20  
  Purchase #6 $48.40   9.466%   $52.9815   $52.98   $53.00   $53.00  
  Purchase #7 $8.70   8.666%   $9.4539   $9.45   $9.45   $9.50  
  7 purchase totals = $558.80       $608.0771   $608.06   $608.05   $608.10