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 $8.35   11.748%   $9.331   $9.33   $9.35   $9.30  
  Purchase #2 $14.66   4.359%   $15.299   $15.30   $15.30   $15.30  
  Purchase #3 $137.81   6.839%   $147.2348   $147.23   $147.25   $147.20  
  Purchase #4 $18.48   12.630%   $20.814   $20.81   $20.80   $20.80  
  Purchase #5 $78.37   4.158%   $81.6286   $81.63   $81.65   $81.60  
  Purchase #6 $101.80   6.300%   $108.2134   $108.21   $108.20   $108.20  
  Purchase #7 $136.43   8.380%   $147.8628   $147.86   $147.85   $147.90  
  7 purchase totals = $495.90       $530.3836   $530.37   $530.40   $530.30