Depending on type of aircraft it is not advisable that you fully look out of the window until end of rotation. On widebodies, your margin between not climbing and striking your tail is so narrow that you have no chance to guess your pitch by looking out of the window - you basically only see your dashboard as a line witout horizontal reference of the wings or other parts of the aircraft.
That's because you are most likely seated too low.
With the recommended seating position to obtain optimum eye reference position you should have "sight along the upper surface of the glare shield with a small amount of the airplane nose structure visible" (according to the NG FCOM, most likely your aircraft will have a similar recommendation).
So you should have the nose visible and in that case it is very easy to judge the angle (and rate of change) compared either to horizon or other ground reference - far easier than on a 2D instrument, depth perception does wonders in judging speeds, distances and accelerations.
When we learned to fly a SEP straight and level, we looked at nose versus the horizon - basic flying. Obviously there are situations where we have to use instruments as primary reference when flying airliners, such as when the nose is pointed 20 degs upwards and it's no longer possible to see the horizon, but that doesn't mean we have to discard basic flying techniques all together, in particular not in situations where they are superior.
With the "look out the window during the whole rotation" brigade I have always wondered just exactly they thought that they were looking at during a low visibility take-off!
Low vis takeoffs are another story and just because we use a special technique, doesn't mean we have to apply that to the 95% other takeoffs we make - we don't do auto lands all the time either, do we?
Besides, described technique can be used when the weather misty too, you don't need 10+ km visibility to have a reference point on ground when at such a low altitude. If the cloud cover is low, switch to instruments earlier - common sense?