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 $241.56   13.258%   $273.586   $273.59   $273.60   $273.60  
  Purchase #2 $209.31   12.320%   $235.097   $235.10   $235.10   $235.10  
  Purchase #3 $27.77   3.566%   $28.7603   $28.76   $28.75   $28.80  
  Purchase #4 $248.76   5.165%   $261.6085   $261.61   $261.60   $261.60  
  Purchase #5 $178.57   10.805%   $197.8645   $197.86   $197.85   $197.90  
  Purchase #6 $201.25   5.704%   $212.7293   $212.73   $212.75   $212.70  
  Purchase #7 $118.72   10.647%   $131.3601   $131.36   $131.35   $131.40  
  7 purchase totals = $1,225.94       $1,341.0057   $1,341.01   $1,341.00   $1,341.10