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 $16.45   8.132%   $17.7877   $17.79   $17.80   $17.80  
  Purchase #2 $81.40   11.551%   $90.8025   $90.80   $90.80   $90.80  
  Purchase #3 $142.38   3.214%   $146.9561   $146.96   $146.95   $147.00  
  Purchase #4 $66.25   4.664%   $69.3399   $69.34   $69.35   $69.30  
  Purchase #5 $23.99   7.724%   $25.843   $25.84   $25.85   $25.80  
  Purchase #6 $121.51   11.590%   $135.593   $135.59   $135.60   $135.60  
  Purchase #7 $78.65   10.148%   $86.6314   $86.63   $86.65   $86.60  
  7 purchase totals = $530.63       $572.9536   $572.95   $573.00   $572.90