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 $205.97   12.558%   $231.8357   $231.84   $231.85   $231.80  
  Purchase #2 $136.63   7.315%   $146.6245   $146.62   $146.60   $146.60  
  Purchase #3 $61.51   7.708%   $66.2512   $66.25   $66.25   $66.30  
  Purchase #4 $246.96   3.187%   $254.8306   $254.83   $254.85   $254.80  
  Purchase #5 $103.34   5.630%   $109.158   $109.16   $109.15   $109.20  
  Purchase #6 $5.51   11.291%   $6.1321   $6.13   $6.15   $6.10  
  Purchase #7 $198.84   8.676%   $216.0914   $216.09   $216.10   $216.10  
  7 purchase totals = $958.76       $1,030.9235   $1,030.92   $1,030.95   $1,030.90