How bad is this if you don't stare at the wiki/walkthrough the whole time? Will you miss a lot or get locked out of things?
While everything has been answered already, it hasn't been answered by everybody, as they say, so my two cents:
You will most probably miss quite a bit of content with specific girls, although not necessarily the mentioned 35%. You may also miss a girl or three because they will leave you for good if you do stuff wrongly. How much will depend a bit on luck and a bit on your playstyle - if you are normally more into following the walkthrough it will be a bit more than if you are somebody who just explores locations on the map and raises stats a bit even if not asked for at the moment.
The experience will also depend on how important a "full first run" (you cannot get everything in one playthrough anyway, so I mean as much as possible) compared to "finding your own way" is to you personally. Some stuff in my opinion is better to experience first hand than to be spoilered when experiencing it the first time. But even the Wiki itself will be confusing the first walkthrough and you wouldn't even know where to look.
My suggestion:
Play a first playthrough as you feel but
- pick Martial Arts as a character trait - it may arguably not be the best and can be learned later, but it can prevent a sudden game over at certain points
- raise your stats a little
- if you want a mostly complete set of girls: allow Shiori to call you "Sir", go to the Cinema early on, concentrate on Sasha in the beginning until you have met her ex, Picking Guitar Play as a skill also helps but is not necessary
- avoid the dark alley for a while
- if everything is going well and you want to continue,
You should get a feel for the game and at one point be able to make an informed decision whether you want to continue like this. However, if you do not want to continue but have enough fun to want to restart the game and do stuff better, consider marrying one of the girls left in the game (finishing her storyline, buying a ring and proposing) to have a "New Game +" with all your accumulated stats and money instead of starting empty.
