Yeah, that might have been a bit optimistic. Mostly because at the time my experience with being measured always underestimated my true fat percentage. For example one of my friend who is solid coach otherwise measured me at 4.1% which I was, to my estimation, about 10-12lbs from really being in good shape.
I will also admit that at the time Charles Poliquin was my greatest influence and he is the one who talked about a guy never needing to be higher than 10%, so I ran with that figure.
That having been said I believe that a true 12% body fat is reasonably easy to maintain IF you are fairly lean to start with.
It really is an individual thing. Take someone who has been in the 20s% all his life... he diet down hard to finally reach 10%. Well, for that guy maintaining 10% will be much harder than for someone who has never been above 10-12% his whole life... the body will strive to get back to its natural set-point. If you've been 20% for 10 years and 10% for 1 month guess where your set-point will be?
So for that guy it might be acceptable to get up to 14-15% when trying toi build a lot of muscle. But the longer he stays lean, the lower his set-point can become.
So if he goes down to 10% it is expected that his BF will go up a bit when trying toi build muscle. But he should definetly avoid going back up as high as it was.. the goal over time would be that each "cut/gain" cycle would leave his BF percentage slighlt lower than the preceding one, gradually adjusting his set-point down.
Another element to consider is what are you comfortable with? Some people do not mind being 15% body fat, while others freak out if they get over 9%. I think that it is important never to reach a point where you do not like what you see.
Finally, the easier it is for you to lose fat (physically and psychologically) the less problematic it is if you gain fat because you know you'll have no problem melting it off when needed. If losing fat is very hard for you, if it is slow and psychologically draining then it is best to avoid pilling on too much of it.
Take me for example... while losing fat is not that hard for me, I tend to lose muscle relatively fast when I diet down. So if I have to diet down for more than 4 weeks to get the leanness I want, I know I will lose muscle. So I need to avoid adding too much fat.
A friend of mine and pro bodybuilder is amazing when dieting, he is a robot, never strays aways from his diet until the job is done and has a very stable muscle mass. He competes at 205lbs but goes up to 245lbs off-season because he knows he wont have trouble dieting it off.