For me I'm always under 15%. Currently at 10. But it's not a big deal one way or the other. I'm not about to micro manage something like that. But I also make a habit not to overspend. 
70~90%% util for a year now (long story, don't wanna get into it), and I haven't heard a peep from any of these companies.
Just recently paid off three, however, and I haven't even been balance chased.
incomprehensible, man.
I want them to take adverse action.
I. want. to be. nailed.
Why won't they?
Am I not bad enough?
Why doesn't Chase pay attention to me? 
@Stralem wrote:70~90%% util for a year now (long story, don't wanna get into it), and I haven't heard a peep from any of these companies.
Just recently paid off three, however, and I haven't even been balance chased.
incomprehensible, man.
I want them to take adverse action.
I. want. to be. nailed.
Why won't they?
Am I not bad enough?
Why doesn't Chase pay attention to me?
I don't see your income listed here and your history with Chase, but maybe they are the reasons?
I'm sure by now people here realize that utilization is not the sole metric, not even the credit report; lenders also look at income, current employer, other obligations etc.
I hear Barclays horror stories on these boards and I even panicked for a while thinking Barclays will balance chase me the moment I paid down their 80-90% balance on the $3k SMMC that I had been carrying over 4-5 months. I used to pay down 3-400 and then charge it back again, groceries and gas, 0% APR. No AA once I paid it down, instead, received a $3k auto CLI in the immediate statement. Discover often offers SP and auto CLIs with high enough balances, no problem. My friend got an auto CLI on his Chase Freedom from $5.5 to $10k with a $4k balance during 0% APR; it's his go-to card. I'm sure we'll come across similar stories here; not only lenders don't go around doing AA like crazy, sometimes they reward heavy usage and carrying balances.
Now when I was at 80% util with Barclays, their CL was less than 10% of my income, and was a small number to start with. The bank knew I could pay it off over time, and saw me making regular payments, even if the balance was not going down. Had that CL been $20k, with my income, the bank would have thought twice and most likely closed the card or taken some other AA. On the other extreme, if the CL was low, and I was purchasing as well as paying many times my income, Barclays would have thought I was MSing and then taken AA, or if I was lucky, done some kind of FR (dunno if it happens with them). Right now, my total CL is more than double my annual individual salary. Even if I let 30% post (which is considered safe territory) across the board, I'm playing with fire.
Anyway, I guess the point is that one man's 50% is another man's, well, no big deal. Just like most other things in credit, you need to consider the full picture. Banks don't mind carrying a balance; as long as the balance is something that the bank thinks you'll be able to pay off. On some cards, it may be 90%, on some others, even 40% may be a big deal.