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 $239.70   13.244%   $271.4459   $271.45   $271.45   $271.40  
  Purchase #2 $31.34   11.230%   $34.8595   $34.86   $34.85   $34.90  
  Purchase #3 $188.77   4.551%   $197.3609   $197.36   $197.35   $197.40  
  Purchase #4 $46.94   5.652%   $49.593   $49.59   $49.60   $49.60  
  Purchase #5 $58.54   4.177%   $60.9852   $60.99   $61.00   $61.00  
  Purchase #6 $172.25   12.691%   $194.1102   $194.11   $194.10   $194.10  
  Purchase #7 $188.95   3.850%   $196.2246   $196.22   $196.20   $196.20  
  7 purchase totals = $926.49       $1,004.5793   $1,004.58   $1,004.55   $1,004.60