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 $242.47   12.299%   $272.2914   $272.29   $272.30   $272.30  
  Purchase #2 $229.45   5.676%   $242.4736   $242.47   $242.45   $242.50  
  Purchase #3 $229.29   10.478%   $253.315   $253.32   $253.30   $253.30  
  Purchase #4 $70.93   7.351%   $76.1441   $76.14   $76.15   $76.10  
  Purchase #5 $126.54   13.402%   $143.4989   $143.50   $143.50   $143.50  
  Purchase #6 $97.88   11.423%   $109.0608   $109.06   $109.05   $109.10  
  Purchase #7 $246.61   3.817%   $256.0231   $256.02   $256.00   $256.00  
  7 purchase totals = $1,243.17       $1,352.8069   $1,352.80   $1,352.75   $1,352.80