I prefer to think of the few techniques we practice that start with a move from nagerather than ukeas really being isolated out of a longer exchange of movements, in which uke had originally attacked, and then an interplay of moves and responses eventually leads to a situation in which nage is now striking and uke responds then nage does the technique. Why? For some philosophical and some very practical reasons.
First, for everyone saying 'well, nage just does shomenuchi and then uke blocks and nage does ikkyo' think about this: nage does shomenuchi, and then uke just does shomenchi ikkyo as we are all taught. The only reason uke blocks vs doing a technique is because he has to in order for the partners to do what was just shown. So I would prefer not to actually be attacking before I get attacked, as that gives the opening to my partner.
Another example is the way my current place teaches kata tori ikkyo (nikyo, etc). After uke grabs nage's forward shoulder, nage moves forward into uke's free hand. Being a smaller nage, I think it is dangerous to get my face within uke's striking distance before his is within mine, and would prefer to either move beofre he can grab my shoulder, or move to the outside to unbalance him (and keep his free hand out of reach of my face). I was told by one of our senior instructors that nage had to move straight forward in towards uke's free hand, and what protected nage was nage hits uke before uke can hit nage. This I find unacceptable for two reasons: first, practically speaking if I am going to trade punches with an uke twice my size, I don't think I will be likely to get the best of the deal. Second, if I was interested in trading punches I be in boxing.
So, I think in practice there are techniques to be learned that are started by nage, but I don't think they are meant as nage initiating the physical contact but are meant to show what would be the middle or end of several moves on both people's parts. Or at least, that is how I look at it; I don't see the point in attacking someone who has not attacked me yet. And I don't think you can ever be 100% sure 'he is going to attack'. I have talked a lot of very belligerant drunks and not too drunk folks out of attacking in my ER, when others were certain they could not be pacified.