A tone may, upon certain instances, take on a pitch different from its classic pitch. That is to say, while a pitch is absolute (in sound frequency), a tone is not.
中餐 may change from the classic zhōng˥ ˥ cān˥ ˥ to zhōng˥ ˥ cān˧ ˧ when 中 is intended to be stressed by the speaker (not necessarily in a question).
葡萄牙 may change from the classic Pú˧ ˥ táo˧ ˥ yá˧...
拜拜 is not casualness and/or childishness,it is a more friendly way to express goodbye ,and majority use between people who have good relation,or see somebody for the first time and build a friendship each other.
So does it in Taiwan I think:)
Aside from @L Parker's excellent answer, I can provide some empirical evidence.
In my experience, the second tone is pronounced with a clear 35 tone when the word is:
a single-character word, where a clear 2nd tone helps disambiguate (薄，毛，抗)
followed by a 5th tone (鼻子 bi2zi0，猴子 hou2zi0，葡萄 pu2tao0)
at the end of sentence
Then you can have altered ...