It's perfectly reasonable to do that. I, sadly just put a new rear tire on my bike for IM this weekend even though the existing tire(Conti GP3000's) only has around 500 miles on it. Why? Because it has noticable nicks and cuts. A few of them are pretty deep, and I'd feel pretty stupid if I got a blowout because I didn't want to spend $35 on a new tire. Is a blowout likely? No. But $35 is cheap insurance with race day approaching.

If the treads look bald to you, replace them. If they show a lot of cuts, replace them. If you aren't sure, take your bike to the LBS and ask them for their opinion. Mine is pretty good about telling me when a part is, or isn't, in need of replacement(and they usually say the part in question is fine.)

Keep in mind, even good tires may wear out quickly. Often, very high performance tires have a life of just a few hundred miles. It's the trade-off for exceptional grip and handling. My last tires, Conti Ultra 3000's, were okay tires, but lasted a few thousand miles, compared to the GP3000's, which show significant wear after just a few hundred. (I should clarify: The GP3000's still have plenty of tread left, but being a softer compound than the Ultras, they show wear sooner.)

Good luck at your race.

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