shading them won't hurt, as long as you give them room to breathe. most units need about 4' around them and 6-8' or more above them, though yours could vary.

i have seen "proof" both that screening them makes them work more efficiently and that screenign them cause worse results or no change. all depends on what data you read.

i can tell you for a fact that when my trees shade the unit during mid afternoon it cools better than during lunch time when it is not shaded, and mid afternoon is hotter than lunchtime here.

There is no doubt, shading the unit will help it's performance as long as you follow davidandkasies good advice. Make sure anything that is shading the condensing unit, does not block air flow or recycle the air that goes through the condensing coils. I like the 4 foot clearance idea for 2 reasons, it allows plenty of air flow so make sure if you put viney things on the trellis, they don't get out of control and invade that space. I've had to "weed" many condensing units that have failed or don't perform. Also, your service man needs to get in there to work or, to dodge wasps, snakes and other critters. I'm 6-5 and weigh 225 pounds and some of the units I've worked on made me feel like I was doing jumping jacks in a phone booth. Some of the other guys that read the thread can relate to their face against a wall and one eye peeking into a control box.