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 $18.49   9.669%   $20.2778   $20.28   $20.30   $20.30  
  Purchase #2 $16.76   11.351%   $18.6624   $18.66   $18.65   $18.70  
  Purchase #3 $235.65   8.461%   $255.5883   $255.59   $255.60   $255.60  
  Purchase #4 $30.90   7.131%   $33.1035   $33.10   $33.10   $33.10  
  Purchase #5 $145.29   12.495%   $163.444   $163.44   $163.45   $163.40  
  Purchase #6 $217.49   5.258%   $228.9256   $228.93   $228.95   $228.90  
  Purchase #7 $32.36   7.121%   $34.6644   $34.66   $34.65   $34.70  
  7 purchase totals = $696.94       $754.6660   $754.66   $754.70   $754.70