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 $11.91   9.325%   $13.0206   $13.02   $13.00   $13.00  
  Purchase #2 $140.55   11.670%   $156.9522   $156.95   $156.95   $157.00  
  Purchase #3 $144.76   8.768%   $157.4526   $157.45   $157.45   $157.50  
  Purchase #4 $12.97   8.750%   $14.1049   $14.10   $14.10   $14.10  
  Purchase #5 $161.38   6.163%   $171.3258   $171.33   $171.35   $171.30  
  Purchase #6 $74.91   10.387%   $82.6909   $82.69   $82.70   $82.70  
  Purchase #7 $225.85   4.431%   $235.8574   $235.86   $235.85   $235.90  
  7 purchase totals = $772.33       $831.4044   $831.40   $831.40   $831.50