I have a "girl" hair bucket and a "boy" hair bucket. My girl hair bucket I keep upstairs in my bathroom because that is usually where I do her hair. I could put it in the kid bathroom...but I believe you should store stuff where you actually use it.
In keeping with this pattern of thought is why my "boy" hair bucket is in my kitchen pantry..."Why the pantry?" you might be asking yourself...well because I usually do my boy's hair after they eat breakfast right before they walk out the door. So to be "efficiently mommy organized" I don't put it in their bathroom...I put it where I use it!