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 $18.84   10.331%   $20.7864   $20.79   $20.80   $20.80  
  Purchase #2 $41.72   4.350%   $43.5348   $43.53   $43.55   $43.50  
  Purchase #3 $221.64   6.532%   $236.1175   $236.12   $236.10   $236.10  
  Purchase #4 $46.56   8.650%   $50.5874   $50.59   $50.60   $50.60  
  Purchase #5 $80.71   9.785%   $88.6075   $88.61   $88.60   $88.60  
  Purchase #6 $8.37   10.472%   $9.2465   $9.25   $9.25   $9.20  
  Purchase #7 $130.95   6.433%   $139.374   $139.37   $139.35   $139.40  
  7 purchase totals = $548.79       $588.2541   $588.26   $588.25   $588.20