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 $50.58   13.838%   $57.5793   $57.58   $57.60   $57.60  
  Purchase #2 $82.86   11.800%   $92.6375   $92.64   $92.65   $92.60  
  Purchase #3 $117.68   9.588%   $128.9632   $128.96   $128.95   $129.00  
  Purchase #4 $238.54   8.872%   $259.7033   $259.70   $259.70   $259.70  
  Purchase #5 $27.54   12.199%   $30.8996   $30.90   $30.90   $30.90  
  Purchase #6 $23.78   9.216%   $25.9716   $25.97   $25.95   $26.00  
  Purchase #7 $64.89   8.482%   $70.394   $70.39   $70.40   $70.40  
  7 purchase totals = $605.87       $666.1485   $666.14   $666.15   $666.20