Momto8 -
Generally, we were at the parks at opening time and were catching buses at their most crowded when leaving the resort in the morning. The staff was very well organized and had everything under control. On most mornings, there was a person with a walkie talkie and clip board who was "directing" people traffic and keeping everything organized. Very often, if one bus filled up to capacity and left many people standing there, an empty bus would pull up 1-2 minutes behind it!
The longest waits we had were on returning to the hotel from the parks (both in mid-afternoon and night-time). That is where we would wait 5-7 minutes for the buses. Sometimes we were the only ones in the line for the hotel, sometimes we were in a very long line (after the MK afternoon parade, and after a terrential downpour in AK). But even then, if one bus filled up, another empty one came right along behind it.
Truly -- buses were no problem at all! (and everyone was so nice, if we were going to have to stand, often someone would give up a seat to me since I had both my 2 year old and 4 year old with me).