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 $21.25   8.366%   $23.0278   $23.03   $23.05   $23.00  
  Purchase #2 $34.34   6.174%   $36.4602   $36.46   $36.45   $36.50  
  Purchase #3 $217.80   6.314%   $231.5519   $231.55   $231.55   $231.60  
  Purchase #4 $68.53   12.412%   $77.0359   $77.04   $77.05   $77.00  
  Purchase #5 $136.72   8.672%   $148.5764   $148.58   $148.60   $148.60  
  Purchase #6 $42.42   10.142%   $46.7222   $46.72   $46.70   $46.70  
  Purchase #7 $75.30   7.192%   $80.7156   $80.72   $80.70   $80.70  
  7 purchase totals = $596.36       $644.0900   $644.10   $644.10   $644.10