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 $126.63   9.752%   $138.979   $138.98   $139.00   $139.00  
  Purchase #2 $129.46   9.514%   $141.7768   $141.78   $141.80   $141.80  
  Purchase #3 $129.53   4.837%   $135.7954   $135.80   $135.80   $135.80  
  Purchase #4 $225.45   7.369%   $242.0634   $242.06   $242.05   $242.10  
  Purchase #5 $5.73   13.789%   $6.5201   $6.52   $6.50   $6.50  
  Purchase #6 $125.89   11.625%   $140.5247   $140.52   $140.50   $140.50  
  Purchase #7 $9.92   12.290%   $11.1392   $11.14   $11.15   $11.10  
  7 purchase totals = $752.61       $816.7986   $816.80   $816.80   $816.80