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 $123.64   9.303%   $135.1422   $135.14   $135.15   $135.10  
  Purchase #2 $44.86   9.170%   $48.9737   $48.97   $48.95   $49.00  
  Purchase #3 $62.69   8.689%   $68.1371   $68.14   $68.15   $68.10  
  Purchase #4 $21.55   11.190%   $23.9614   $23.96   $23.95   $24.00  
  Purchase #5 $227.51   4.395%   $237.5091   $237.51   $237.50   $237.50  
  Purchase #6 $127.31   4.860%   $133.4973   $133.50   $133.50   $133.50  
  Purchase #7 $10.45   12.624%   $11.7692   $11.77   $11.75   $11.80  
  7 purchase totals = $618.01       $658.9900   $658.99   $658.95   $659.00