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 $9.81   6.225%   $10.4207   $10.42   $10.40   $10.40  
  Purchase #2 $129.65   8.630%   $140.8388   $140.84   $140.85   $140.80  
  Purchase #3 $97.79   9.836%   $107.4086   $107.41   $107.40   $107.40  
  Purchase #4 $204.95   7.392%   $220.0999   $220.10   $220.10   $220.10  
  Purchase #5 $57.59   3.131%   $59.3931   $59.39   $59.40   $59.40  
  Purchase #6 $33.59   6.719%   $35.8469   $35.85   $35.85   $35.80  
  Purchase #7 $237.41   12.336%   $266.6969   $266.70   $266.70   $266.70  
  7 purchase totals = $770.79       $840.7049   $840.71   $840.70   $840.60