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 $226.56   13.246%   $256.5701   $256.57   $256.55   $256.60  
  Purchase #2 $58.88   4.719%   $61.6585   $61.66   $61.65   $61.70  
  Purchase #3 $13.26   12.592%   $14.9297   $14.93   $14.95   $14.90  
  Purchase #4 $28.39   6.272%   $30.1706   $30.17   $30.15   $30.20  
  Purchase #5 $168.69   3.107%   $173.9312   $173.93   $173.95   $173.90  
  Purchase #6 $196.32   6.716%   $209.5049   $209.50   $209.50   $209.50  
  Purchase #7 $127.92   7.674%   $137.7366   $137.74   $137.75   $137.70  
  7 purchase totals = $820.02       $884.5016   $884.50   $884.50   $884.50