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 $100.71   4.574%   $105.3165   $105.32   $105.30   $105.30  
  Purchase #2 $145.74   6.533%   $155.2612   $155.26   $155.25   $155.30  
  Purchase #3 $208.58   10.706%   $230.9106   $230.91   $230.90   $230.90  
  Purchase #4 $111.91   5.293%   $117.8334   $117.83   $117.85   $117.80  
  Purchase #5 $122.42   10.551%   $135.3365   $135.34   $135.35   $135.30  
  Purchase #6 $9.51   13.632%   $10.8064   $10.81   $10.80   $10.80  
  Purchase #7 $81.36   3.553%   $84.2507   $84.25   $84.25   $84.30  
  7 purchase totals = $780.23       $839.7153   $839.72   $839.70   $839.70