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 $16.43   12.314%   $18.4532   $18.45   $18.45   $18.50  
  Purchase #2 $16.87   3.224%   $17.4139   $17.41   $17.40   $17.40  
  Purchase #3 $219.59   4.859%   $230.2599   $230.26   $230.25   $230.30  
  Purchase #4 $63.39   11.120%   $70.439   $70.44   $70.45   $70.40  
  Purchase #5 $11.42   12.600%   $12.8589   $12.86   $12.85   $12.90  
  Purchase #6 $138.51   10.316%   $152.7987   $152.80   $152.80   $152.80  
  Purchase #7 $134.37   3.137%   $138.5852   $138.59   $138.60   $138.60  
  7 purchase totals = $600.58       $640.8088   $640.81   $640.80   $640.90