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 $100.50   6.225%   $106.7561   $106.76   $106.75   $106.80  
  Purchase #2 $77.76   12.494%   $87.4753   $87.48   $87.50   $87.50  
  Purchase #3 $159.68   4.466%   $166.8113   $166.81   $166.80   $166.80  
  Purchase #4 $45.89   5.441%   $48.3869   $48.39   $48.40   $48.40  
  Purchase #5 $176.71   6.776%   $188.6839   $188.68   $188.70   $188.70  
  Purchase #6 $137.43   5.626%   $145.1618   $145.16   $145.15   $145.20  
  Purchase #7 $73.30   6.179%   $77.8292   $77.83   $77.85   $77.80  
  7 purchase totals = $771.27       $821.1045   $821.11   $821.15   $821.20