I think someone in another question here (or on another site?) said he was told by his "chassan teacher" not to look at his wife unclothed. It seems like this is not the real halacha to me, or at least not everyone holds it. What are the opinions?
It is definitely allowed according to halachah; this teacher was probably speaking about what he felt was the pious and holy way to behave. But the truth is that one must be extremely careful when trying to act "beyond the letter of the law," that he should not damage his Sholom Bayis in the process. The Chazon Ish in his "Iggeres Kodesh" (cited in Mishkan Yisroel page 50) puts it as follows:
ההסתכלות באשתו מותר מפני שרצון התורה באהבה ביניהם, והאהבה היא ההכנה למצות עונה העתידה
In other words, the primary and overriding goal is to foster a healthy and comfortable relationship. Any practices that are done on account of "perishus" and "kedushah" should not detract from that.
(I would hope that the aforementioned chassan teacher conveyed these nuances.)
If [a husband] had relations with his wife and waited several days, and [afterwards,] claimed that he discovered a blemish only then, his words are disregarded. [This applies] even if [the blemish] is in the folds [of the woman's skin] (meaning, for example, under the armpit or under the breasts and the like) or on the sole of her foot. [The rationale is that] we presume that a man will not drink from a cup unless he checks it well first. [Therefore,] we assume that he knew [of the blemish] and accepted it.