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 $61.47   8.116%   $66.4589   $66.46   $66.45   $66.50  
  Purchase #2 $53.97   5.861%   $57.1332   $57.13   $57.15   $57.10  
  Purchase #3 $81.60   7.510%   $87.7282   $87.73   $87.75   $87.70  
  Purchase #4 $250.64   6.762%   $267.5883   $267.59   $267.60   $267.60  
  Purchase #5 $181.25   4.108%   $188.6958   $188.70   $188.70   $188.70  
  Purchase #6 $123.28   12.712%   $138.9514   $138.95   $138.95   $139.00  
  Purchase #7 $34.29   10.528%   $37.9001   $37.90   $37.90   $37.90  
  7 purchase totals = $786.50       $844.4559   $844.46   $844.50   $844.50