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 $15.57   4.326%   $16.2436   $16.24   $16.25   $16.20  
  Purchase #2 $8.73   3.425%   $9.029   $9.03   $9.05   $9.00  
  Purchase #3 $114.97   12.766%   $129.6471   $129.65   $129.65   $129.60  
  Purchase #4 $159.74   10.467%   $176.46   $176.46   $176.45   $176.50  
  Purchase #5 $222.49   3.630%   $230.5664   $230.57   $230.55   $230.60  
  Purchase #6 $152.94   8.821%   $166.4308   $166.43   $166.45   $166.40  
  Purchase #7 $102.46   7.491%   $110.1353   $110.14   $110.15   $110.10  
  7 purchase totals = $776.90       $838.5122   $838.52   $838.55   $838.40