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 $61.93   6.612%   $66.0248   $66.02   $66.00   $66.00  
  Purchase #2 $72.74   8.210%   $78.712   $78.71   $78.70   $78.70  
  Purchase #3 $187.87   13.276%   $212.8116   $212.81   $212.80   $212.80  
  Purchase #4 $115.93   5.493%   $122.298   $122.30   $122.30   $122.30  
  Purchase #5 $51.32   10.188%   $56.5485   $56.55   $56.55   $56.50  
  Purchase #6 $198.52   13.669%   $225.6557   $225.66   $225.65   $225.70  
  Purchase #7 $216.27   13.660%   $245.8125   $245.81   $245.80   $245.80  
  7 purchase totals = $904.58       $1,007.8631   $1,007.86   $1,007.80   $1,007.80