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 $19.97   5.158%   $21.0001   $21.00   $21.00   $21.00  
  Purchase #2 $83.84   3.841%   $87.0603   $87.06   $87.05   $87.10  
  Purchase #3 $54.81   10.766%   $60.7108   $60.71   $60.70   $60.70  
  Purchase #4 $229.72   11.175%   $255.3912   $255.39   $255.40   $255.40  
  Purchase #5 $17.67   13.621%   $20.0768   $20.08   $20.10   $20.10  
  Purchase #6 $246.68   6.440%   $262.5662   $262.57   $262.55   $262.60  
  Purchase #7 $74.62   12.117%   $83.6617   $83.66   $83.65   $83.70  
  7 purchase totals = $727.31       $790.4671   $790.47   $790.45   $790.60