Tm's of oligo dT primers

[Steven Sullivan]

Do the calculated Tm's of oligo dT-containing primers  (15-20 T's) bear 
*any* resemblance to their actual performance at different PCR annealing 
temperatures, in anyone's experience?  

And (I know someone asked this recently, but I've lost the thread) when 
factoring salt concentration into a Tm calculation, does one include all 
cationic species (K+, Mg++, Tris?) in the calculation or just Mg++?