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 $16.46   12.525%   $18.5216   $18.52   $18.50   $18.50  
  Purchase #2 $174.84   12.629%   $196.9205   $196.92   $196.90   $196.90  
  Purchase #3 $63.37   6.758%   $67.6525   $67.65   $67.65   $67.70  
  Purchase #4 $210.86   13.686%   $239.7183   $239.72   $239.70   $239.70  
  Purchase #5 $248.93   7.659%   $267.9955   $268.00   $268.00   $268.00  
  Purchase #6 $23.69   4.232%   $24.6926   $24.69   $24.70   $24.70  
  Purchase #7 $155.35   9.458%   $170.043   $170.04   $170.05   $170.00  
  7 purchase totals = $893.50       $985.5440   $985.54   $985.50   $985.50