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 $56.68   6.174%   $60.1794   $60.18   $60.20   $60.20  
  Purchase #2 $222.42   8.168%   $240.5873   $240.59   $240.60   $240.60  
  Purchase #3 $63.63   8.331%   $68.931   $68.93   $68.95   $68.90  
  Purchase #4 $52.81   3.224%   $54.5126   $54.51   $54.50   $54.50  
  Purchase #5 $62.51   9.366%   $68.3647   $68.36   $68.35   $68.40  
  Purchase #6 $20.74   3.713%   $21.5101   $21.51   $21.50   $21.50  
  Purchase #7 $77.67   9.216%   $84.8281   $84.83   $84.85   $84.80  
  7 purchase totals = $556.46       $598.9132   $598.91   $598.95   $598.90