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 $9.93   7.134%   $10.6384   $10.64   $10.65   $10.60  
  Purchase #2 $112.99   8.174%   $122.2258   $122.23   $122.25   $122.20  
  Purchase #3 $190.88   7.156%   $204.5394   $204.54   $204.55   $204.50  
  Purchase #4 $13.42   12.502%   $15.0978   $15.10   $15.10   $15.10  
  Purchase #5 $130.37   10.485%   $144.0393   $144.04   $144.05   $144.00  
  Purchase #6 $153.31   7.289%   $164.4848   $164.48   $164.50   $164.50  
  Purchase #7 $7.69   3.843%   $7.9855   $7.99   $8.00   $8.00  
  7 purchase totals = $618.59       $669.0110   $669.02   $669.10   $668.90