you can do it either way, the signs will work themselves out. i did it with out of the node positive.
if you do it into the node positive, then you would have to negate ALL those terms, in which case all the negative signs would drop out anyway. sounds like you are getting the signs wrong for the current in the terms other than the current sources.
for instance, a/4 refers to the current going OUT of the node a. to get the current into the node you would have to use -a/4.