As a journalist I am pretty used to taping conversations etc. (and with the phone taping laws in the UK). Taping your own thoughts etc. is a nice idea, although transcribing (if you intend to write them down later) can take a while. It can be useful to record where you take photos though (just see what no. photo you are on, and there you go).

As for the taping of guided tours... I would suggest that you asked permission first. I am sure it would almost always be given, but it is courteous at least and, depending on the country you might be in danger of breaking the law, so asking for permission removes any of the danger of sitting in front of a pedantic off-duty policeman!

Jonathan London/Duesseldorf