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 $24.68   12.579%   $27.7845   $27.78   $27.80   $27.80  
  Purchase #2 $57.67   4.733%   $60.3995   $60.40   $60.40   $60.40  
  Purchase #3 $151.56   13.264%   $171.6629   $171.66   $171.65   $171.70  
  Purchase #4 $192.38   3.583%   $199.273   $199.27   $199.25   $199.30  
  Purchase #5 $58.94   4.483%   $61.5823   $61.58   $61.60   $61.60  
  Purchase #6 $52.83   10.603%   $58.4316   $58.43   $58.45   $58.40  
  Purchase #7 $242.52   4.217%   $252.7471   $252.75   $252.75   $252.70  
  7 purchase totals = $780.58       $831.8809   $831.87   $831.90   $831.90