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 $230.54   12.613%   $259.618   $259.62   $259.60   $259.60  
  Purchase #2 $90.67   5.261%   $95.4401   $95.44   $95.45   $95.40  
  Purchase #3 $89.84   4.624%   $93.9942   $93.99   $94.00   $94.00  
  Purchase #4 $41.83   12.379%   $47.0081   $47.01   $47.00   $47.00  
  Purchase #5 $21.66   11.280%   $24.1032   $24.10   $24.10   $24.10  
  Purchase #6 $138.58   7.851%   $149.4599   $149.46   $149.45   $149.50  
  Purchase #7 $242.93   11.721%   $271.4038   $271.40   $271.40   $271.40  
  7 purchase totals = $856.05       $941.0273   $941.02   $941.00   $941.00