I think it is pretty important to cut the lawn at the recommended height. Any taller and the new grass will start laying down which increases it's chance of dying off due to rot and fungal disease since you will still be watering the new seed. The best solution would be to get a reel mower (the one you push) to cut the new fescue. If you can't borrow or rent one, you can buy one for about $150, which is not too much considering what you have spent in time and money on your lawn. This will not suck up the new seeds, and the mower is much lighter than a power mower, reducing damage to the new grass and the new seed. Of course you need to mow before the new seedlings are visible above the soil, as they are very vulnerable to being walked on at that point.

I really appreciate the advice. However I looked into these and I've read that they don't cut very well. A sharp, clean cut would be really important on new fescue no?

What do you think of the string trimmer option? The concern here is it won't have suction which would stand the grass up.

I blew the leaves off today and got a better look at everything. Some of the grass is definitely 3 inches, however most of it is less than 2. And the new seed still hasn't germinated, but its probably close.

If I don't get any feedback, I may just cut it with my push mower in a few sections on a high setting.

I just searched around and read some other threads on here and I found something that might help. One poster said when in a tough spot like this, you could lightly moisten the areas that seed hasn't germinated then mow.

The thought being that the moisture will hopefully help keep the seed in place.

Reel mowers cut very well, especially if they are new or the blades have been recently sharpened. New tall fescue would be no match for a sharp reel mower. If you do use your rotary mower, make sure the blade is sharp, and instead of putting it on the "high setting", put it on a setting that puts the blade at two inches. I know this sounds low, but if you are mowing 3 inch new tall fescue, you want to remove 1/3 of the length, so you want to mow at 2 inches. To do this, measure the distance from the bottom of the blade to the bottom of the mower deck, if it is more than 1/4 inch, then you need to factor this in when you measure the distance from the bottom of the mower deck to a smooth level surface, preferably pavement. I think a light watering would be a good idea to help keep the seeds in place if you use the mower. The string trimmer idea I don't like because it is hard to keep a constant and correct length for the whole area you intend to cut. You can do a lot more harm than good IMO.

I agree with tiemco that the reel mower is the way to go.
I bought the scotts reel mower from home depot just for this purpose. It provides an excellent cut with absolutely no chance of sucking up seed or baby seedlings.

The only downside is it doesn't mulch leaves and it's max height is 3".
So I still plan to use the rotary mower in late fall and when I'm mowing above 3".

Cool. Thanks everyone. I think I will try to buy a rotary then. I'll update in case anyone's interested.