I heard this about tables of 5 too but I didn't have any issues with getting reservations for 5 (or I should say, if a table for 5 wasn't available, there wasn't one for 6 either). I have heard many reports of it, but I didn't have that issue. It does seem to be an issue at certain restaurants though, based on reports here. Here are the ones I didn't have issues with an odd number:
T-Rex
Raglan Road
Be Our Guest (all meals)
Skipper Canteen
Liberty Tree Tavern
Chef Mickey
California Grill
Tusker House
Ohana (both breakfast and dinner)
Sci-Fi Dine In
Mama Melrose
Hollywood & Vine
Whispering Canyon
(no, I didn't keep all these, but these are all reservations I had at one point for this trip, for a family of 5).