In The Merchant Of Venice, did Portia help Bassanio in choosing the lead casket?

No, Portia could not have assisted Bassanio in any way. Before he goes to make his choice, she tells him, in part, at the beginning of Act 3, scene 2 (lines 10-14):

...I could teach you
How to choose right, but I am then forsworn;
So will I never be: so may you miss me;
But if you do, you'll make me wish a sin,
That I had been forsworn.

She means that she could give him a clue or tell him which casket to choose, but she would then be breaking an oath. It is a condition of her father's will that she may not provide any assistance to a suitor for she would then lose her inheritance. Portia says that she will not commit perjury (lie under oath). She also says that if Bassanio should choose the wrong casket, he would make her wish that she had helped him, which would be a sinful thought.

Some interpreters suggest that the introductory lines of the song being played just before Bassanio makes his choice, are supposed to give him a clue since the last word in each of the first two lines rhymes with 'lead':

SONG.
Tell me where is fancy bred,
Or in the heart, or in the head?

This, however, is highly unlikely firstly, for the reason mentioned above - there is just too much at stake for Portia: she will not only lose a fortune if such an indiscretion is reported but will also, as she would have believed, compromise the sanctity of her soul. She would be committing a sin since she had made a sacred vow to not help anyone to make the right choice.

Furthermore, the singers most probably only sing this ditty to add a pleasant mood to what must be a very tense atmosphere if one considers both Portia and Bassanio's wish that he should choose correctly. In addition, Bassanio is too preoccupied with his own thoughts to even listen to the lyrics. He would have no reason to look for extraneous clues anyway for Portia had told him that she would not provide any. 

Interestingly, though, Shakespeare might just have written the first lines to tease the audience and make them believe that Portia was just that bit dishonest.