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 $21.30   7.338%   $22.863   $22.86   $22.85   $22.90  
  Purchase #2 $220.59   4.241%   $229.9452   $229.95   $229.95   $229.90  
  Purchase #3 $112.91   9.206%   $123.3045   $123.30   $123.30   $123.30  
  Purchase #4 $70.89   11.678%   $79.1685   $79.17   $79.15   $79.20  
  Purchase #5 $62.75   9.866%   $68.9409   $68.94   $68.95   $68.90  
  Purchase #6 $131.81   8.658%   $143.2221   $143.22   $143.20   $143.20  
  Purchase #7 $20.34   9.584%   $22.2894   $22.29   $22.30   $22.30  
  7 purchase totals = $640.59       $689.7336   $689.73   $689.70   $689.70