A good set of tools is important to maintaining your equipment. An important thing to have in your tool box is a good set of hex (allen) wrenches. For about 90% of your equipment, you'll need SAE sizes (fractional inch measurements) although a few things will use metric sizes (most noteably, X-ray generators).
While folding sets are convenient and easily stored, I recommend a good set of "L" arm wrenches as these give more consistent leverage and the two different length arms allow for use under different circumstances. The long arm allows greater reach and the short arm provides more leverage for stuck fittings.
Make sure the set you get includes the smallest sizes, down to .050", commonly used on unit holders. They also make hex wrenches with ball ends, these are very nice as you can use them even if you don't have straight line access to a fitting.
A good set will also include a holder so you can keep your wrenches organized to have the correct size readily at hand.
I've had very good success with the Eklind brand, but there are many good tools on the market.