davebaldwin:
(TowerPro SG-90) as a locking pin driver.
By coincidence I have EXACTLY the same Q about EXACTLY the same part for very similar reasons.
First though, are you aware of current draw? Towerpro.com doesn't mention the mA spec. However it's supposed to be comparable to Hitec hs-55 so I went looking for specs. hitecrcd.com does not mention that particular spec. [sigh] I did find 'something' at http://www.servocity.com/html/hs-55_sub-micro.html#.U0jU9Vfwq68 It says 4.8V 'no load operating' 150 mA and 6V 180 mA. My usage has 'very low' load (which is why I'm using this tiny servo) but -with- load might draw -tons- more. Here's a little discussion about that. RC Groups - View Single Post - Calculating Servo Draw or How Can I Tell I Need a UBEC?
A guru at my local RC shop tells a story about a guy who 'thought' he was having slow servo action in flight. Guru asks if he has enough batteries/BEC behind the servos. Guy says sure. Guru say are you SURE. Guy says, well I don't really -know- for sure. Guru puts a flight data recorder in the guy's plane. He was flabbergasted to find that at peak the servos were drawing 90A, overdrawing the batteries and the BEC.
The point being that without actually -measuring- what's going on I wouldn't trust an Arduino board to drive more than 1 or -maybe- 2 of even these tiny micro servos. It's not worth the potential overheating/brownout issues. ('Potential' - no pun intended. 
) I'm using a separate PS because it's absolutely required as I'll have eventually a dozen of these servos running at the same time.
My issue is that the servo jitters when power is supplied, enough so that the locking pin becomes momentarily 'unlocked' ... kind of defeating the point.
I'm going to look into the 4.7K resistor mentioned. Same as you, for me it is absolutely required that there be no motion on power-up.
The main reason for my replying is to suggest taking a look at these little widgets whose purpose -is- 'car door lock actuators': Uneeksupply | eBay Stores Yeah they're 12V but that -might- only mean that they'd operate 'slowly' at 5V. For my purpose I happen to have plenty of 12V lying around so if the resistor doesn't work I'm going to seriously consider these actuators. Yeah in small quantities they're about 50% more than the servo but if it's easier then it's worth it. Yeah they're much larger too, that's another consideration, but easier is always easier. 