Hygro is pretty important, even indoors. Escpecially since I would put one 2nd level, main level, and basement. Basement is really where I would want to track humidity more closely. Might even have one in the garage.
No such thing as too small. What are your thoughts on LCD display, I know that complicates things, but that is one advantage of the OS sesnor. For example my pool one, I (or the wife/kids) can just have a glance and know what the temp is. Rather than going inside and looking on WD or my site. Same for room ones....
Andrew