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 $236.63   12.362%   $265.8822   $265.88   $265.90   $265.90  
  Purchase #2 $147.54   9.258%   $161.1993   $161.20   $161.20   $161.20  
  Purchase #3 $119.84   13.377%   $135.871   $135.87   $135.85   $135.90  
  Purchase #4 $44.73   10.673%   $49.504   $49.50   $49.50   $49.50  
  Purchase #5 $78.58   13.254%   $88.995   $89.00   $89.00   $89.00  
  Purchase #6 $237.68   3.607%   $246.2531   $246.25   $246.25   $246.30  
  Purchase #7 $225.99   13.238%   $255.9066   $255.91   $255.90   $255.90  
  7 purchase totals = $1,090.99       $1,203.6112   $1,203.61   $1,203.60   $1,203.70