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 $214.43   11.711%   $239.5419   $239.54   $239.55   $239.50  
  Purchase #2 $86.47   3.428%   $89.4342   $89.43   $89.45   $89.40  
  Purchase #3 $56.72   13.415%   $64.329   $64.33   $64.35   $64.30  
  Purchase #4 $105.44   9.778%   $115.7499   $115.75   $115.75   $115.70  
  Purchase #5 $82.42   12.550%   $92.7637   $92.76   $92.75   $92.80  
  Purchase #6 $13.66   12.138%   $15.3181   $15.32   $15.30   $15.30  
  Purchase #7 $242.68   8.821%   $264.0868   $264.09   $264.10   $264.10  
  7 purchase totals = $801.82       $881.2236   $881.22   $881.25   $881.10