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 $18.59   12.560%   $20.9249   $20.92   $20.90   $20.90  
  Purchase #2 $38.99   10.587%   $43.1179   $43.12   $43.10   $43.10  
  Purchase #3 $29.83   7.374%   $32.0297   $32.03   $32.05   $32.00  
  Purchase #4 $226.72   13.768%   $257.9348   $257.93   $257.95   $257.90  
  Purchase #5 $27.75   11.675%   $30.9898   $30.99   $31.00   $31.00  
  Purchase #6 $72.90   11.180%   $81.0502   $81.05   $81.05   $81.10  
  Purchase #7 $137.80   8.792%   $149.9154   $149.92   $149.90   $149.90  
  7 purchase totals = $552.58       $615.9627   $615.96   $615.95   $615.90