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 $225.79   12.279%   $253.5148   $253.51   $253.50   $253.50  
  Purchase #2 $164.43   9.382%   $179.8568   $179.86   $179.85   $179.90  
  Purchase #3 $118.88   8.466%   $128.9444   $128.94   $128.95   $128.90  
  Purchase #4 $142.43   12.684%   $160.4958   $160.50   $160.50   $160.50  
  Purchase #5 $57.99   13.513%   $65.8262   $65.83   $65.85   $65.80  
  Purchase #6 $47.42   3.745%   $49.1959   $49.20   $49.20   $49.20  
  Purchase #7 $177.62   5.161%   $186.787   $186.79   $186.80   $186.80  
  7 purchase totals = $934.56       $1,024.6209   $1,024.63   $1,024.65   $1,024.60