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 $5.72   3.766%   $5.9354   $5.94   $5.95   $5.90  
  Purchase #2 $10.71   5.659%   $11.3161   $11.32   $11.30   $11.30  
  Purchase #3 $11.43   9.305%   $12.4936   $12.49   $12.50   $12.50  
  Purchase #4 $237.26   5.233%   $249.6758   $249.68   $249.70   $249.70  
  Purchase #5 $102.51   3.678%   $106.2803   $106.28   $106.30   $106.30  
  Purchase #6 $227.96   4.158%   $237.4386   $237.44   $237.45   $237.40  
  Purchase #7 $15.93   7.482%   $17.1219   $17.12   $17.10   $17.10  
  7 purchase totals = $611.52       $640.2617   $640.27   $640.30   $640.20