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 $59.31   11.224%   $65.967   $65.97   $65.95   $66.00  
  Purchase #2 $90.37   10.472%   $99.8335   $99.83   $99.85   $99.80  
  Purchase #3 $28.31   9.433%   $30.9805   $30.98   $31.00   $31.00  
  Purchase #4 $154.31   5.792%   $163.2476   $163.25   $163.25   $163.20  
  Purchase #5 $93.40   4.551%   $97.6506   $97.65   $97.65   $97.70  
  Purchase #6 $41.59   5.670%   $43.9482   $43.95   $43.95   $43.90  
  Purchase #7 $117.25   12.623%   $132.0505   $132.05   $132.05   $132.10  
  7 purchase totals = $584.54       $633.6779   $633.68   $633.70   $633.70