I don't know a lot about Letro... will it do anything for gyno sides during cycle or will it be necessary to use nolva for that?
Letro works very very quickly because unlike anastrozole, it is specially tailored to penetrate adipose tissue; where most aromatase exists. As such, it kicks in within a day or two and peaks at about 97% aromatase suppression. Anastrozole takes over a week to peak, and maxes out at under 80% suppression.
Still, I wouldn't use Letro as a reactionary tool, I'd just run it straight through. It's far safer to run than Nolva is, and seeing as how you'll be suppressing 97% of all aromatase at a dose of 0.75mg or higher per day, there is pretty much no chance of building enough estrogen to even make you THINK of gyno.
Only one compound I can think of, Letro will do nothing for that Nolva will. Anadrol. That's because Anadrol binds directly to the estrogen receptor without having to aromatize. The thing is, that's where Anadrol gives you the vast majority of its gains. So Nolvadex would only really serve to hinder those gains to a degree.
As far as I see, the only reason to not run Letro is cost. If you're bulking and you don't care much about water retention, it is far more cost effective to just keep some Nolva onhand as a just-in-case, rather than run Letro through your whole cycle.