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 $5.30   7.762%   $5.7114   $5.71   $5.70   $5.70  
  Purchase #2 $15.65   8.821%   $17.0305   $17.03   $17.05   $17.00  
  Purchase #3 $38.51   4.514%   $40.2483   $40.25   $40.25   $40.20  
  Purchase #4 $140.36   5.800%   $148.5009   $148.50   $148.50   $148.50  
  Purchase #5 $61.91   12.726%   $69.7887   $69.79   $69.80   $69.80  
  Purchase #6 $203.76   7.229%   $218.4898   $218.49   $218.50   $218.50  
  Purchase #7 $83.71   4.667%   $87.6167   $87.62   $87.60   $87.60  
  7 purchase totals = $549.20       $587.3863   $587.39   $587.40   $587.30