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 $141.63   12.232%   $158.9542   $158.95   $158.95   $159.00  
  Purchase #2 $186.80   8.335%   $202.3698   $202.37   $202.35   $202.40  
  Purchase #3 $80.46   10.475%   $88.8882   $88.89   $88.90   $88.90  
  Purchase #4 $240.62   4.425%   $251.2674   $251.27   $251.25   $251.30  
  Purchase #5 $15.72   12.447%   $17.6767   $17.68   $17.70   $17.70  
  Purchase #6 $65.25   12.722%   $73.5511   $73.55   $73.55   $73.60  
  Purchase #7 $160.46   8.345%   $173.8504   $173.85   $173.85   $173.90  
  7 purchase totals = $890.94       $966.5578   $966.56   $966.55   $966.80