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 $231.92   3.197%   $239.3345   $239.33   $239.35   $239.30  
  Purchase #2 $154.93   13.560%   $175.9385   $175.94   $175.95   $175.90  
  Purchase #3 $160.52   4.677%   $168.0275   $168.03   $168.05   $168.00  
  Purchase #4 $61.58   5.871%   $65.1954   $65.20   $65.20   $65.20  
  Purchase #5 $5.83   3.688%   $6.045   $6.05   $6.05   $6.00  
  Purchase #6 $141.73   3.233%   $146.3121   $146.31   $146.30   $146.30  
  Purchase #7 $70.60   13.206%   $79.9234   $79.92   $79.90   $79.90  
  7 purchase totals = $827.11       $880.7764   $880.78   $880.80   $880.60