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 $61.79   13.189%   $69.9395   $69.94   $69.95   $69.90  
  Purchase #2 $38.49   6.631%   $41.0423   $41.04   $41.05   $41.00  
  Purchase #3 $149.91   13.853%   $170.677   $170.68   $170.70   $170.70  
  Purchase #4 $195.81   3.270%   $202.213   $202.21   $202.20   $202.20  
  Purchase #5 $104.26   12.222%   $117.0027   $117.00   $117.00   $117.00  
  Purchase #6 $40.66   7.573%   $43.7392   $43.74   $43.75   $43.70  
  Purchase #7 $14.85   3.872%   $15.425   $15.43   $15.45   $15.40  
  7 purchase totals = $605.77       $660.0387   $660.04   $660.10   $659.90