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 $61.84   6.448%   $65.8274   $65.83   $65.85   $65.80  
  Purchase #2 $148.62   12.122%   $166.6357   $166.64   $166.65   $166.60  
  Purchase #3 $85.99   13.112%   $97.265   $97.27   $97.25   $97.30  
  Purchase #4 $53.56   13.352%   $60.7113   $60.71   $60.70   $60.70  
  Purchase #5 $60.51   7.744%   $65.1959   $65.20   $65.20   $65.20  
  Purchase #6 $198.26   7.827%   $213.7778   $213.78   $213.80   $213.80  
  Purchase #7 $144.29   8.843%   $157.0496   $157.05   $157.05   $157.00  
  7 purchase totals = $753.07       $826.4627   $826.48   $826.50   $826.40