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 $60.61   9.312%   $66.254   $66.25   $66.25   $66.30  
  Purchase #2 $81.71   7.218%   $87.6078   $87.61   $87.60   $87.60  
  Purchase #3 $213.64   13.369%   $242.2015   $242.20   $242.20   $242.20  
  Purchase #4 $60.88   11.632%   $67.9616   $67.96   $67.95   $68.00  
  Purchase #5 $141.64   12.211%   $158.9357   $158.94   $158.95   $158.90  
  Purchase #6 $62.69   5.190%   $65.9436   $65.94   $65.95   $65.90  
  Purchase #7 $54.26   13.516%   $61.5938   $61.59   $61.60   $61.60  
  7 purchase totals = $675.43       $750.4980   $750.49   $750.50   $750.50