It would depend on how much top soil you put on. I can't see that a small layer would have a negative impact on the yard, with free rain/watering.

Depending on the amount of levelling needed, it is usually recommended to do it in small steps, if you don't want to kill the grass underneath. And again, the free rain watering by nature will be a good benefit here also.

It will depend on the amount of rain you are getting, since I do recommend not walking too much on your grass when it rains. At least in my yard, it does seem to smash them into the ground more and too much walking along a certain path, when the grass is wet, seems to make it more susceptible to getting destroyed (but it still takes a lot of walking, so it's not to say you can't do some bit of work in between rains).

Sometimes I have even worked at the beginning of the actual rain because I wanted to get some kind of yard work done to take advantage of the rest of the rain period.

You ran out of time 9 months ago for reseeding. If you do it back in early April you would have 100% crabgrass by July. If you do it now, you'll still have 100% crabgrass by July.

Wait. You're in zone 8??
Where do you live?
What kind of grass are you seeding?

Why are you adding topsoil? Because you have a low spot you need to fill?...which is the only correct answer to that question.

Thanks both for the feedback

@dchall_san_antonio: I bought everything now.. will do it anyway and perhaps not get that unfortunate. I live in central Europe. The grass kind.. dunno, about 3 kinds of it. Not one low spot but a myriad of low spots.

Wish me luck!

Where in central Europe?