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 $60.56   6.639%   $64.5806   $64.58   $64.60   $64.60  
  Purchase #2 $43.56   13.309%   $49.3574   $49.36   $49.35   $49.40  
  Purchase #3 $225.75   13.863%   $257.0457   $257.05   $257.05   $257.00  
  Purchase #4 $176.36   11.491%   $196.6255   $196.63   $196.65   $196.60  
  Purchase #5 $118.35   11.184%   $131.5863   $131.59   $131.60   $131.60  
  Purchase #6 $166.44   11.746%   $185.99   $185.99   $186.00   $186.00  
  Purchase #7 $134.38   5.644%   $141.9644   $141.96   $141.95   $142.00  
  7 purchase totals = $925.40       $1,027.1499   $1,027.16   $1,027.20   $1,027.20