There's basically two ways to look at it and John is right - testing is key.
Getting new business is critical obviously and if they're not yet bidding on your brand then I'd focus there BUT it's worse to pay for that first click and then lose the visitor when they find a competitor on the second query. So if your competitors are bidding on your brand you need to grab it, the cost per click is less and you can't afford to lose the conversion after you're paid for the first click but you of course don't want to use some of the limited budget on a second click if you could have had it organically.
Then there comes the discussion over the cost to monitor competitor activities vs the cost of just bidding on the brand. If it costs more to manage it and monitor then just bidding would be the way to go obviously.
Worse case you can test bidding on your brand and monitor the assisted conversions to see how that works out for you.
