On my wife's car I always buy the oil special where you get 5 quarts and a free oil filter for like $11.50. Since I've had the honda I've used castrol gtx 5w-20 and was wondering if it would hurt to just buy whatever the deal is when I change my oil. Sometimes it's pennzoil, castrol, etc. Also, my car has 118k miles and the person at autozone said I should use 10w-30 due to the high mileage.

I prefer to stick with one brand but buying whatever is on sale, providing it meets specs, won't hurt a thing. Unless it is guzzling oil there is no reason to change to 10W30. Use the weight specified for it. If you decide to go to a 30 weight 5W30 would be a better choice.

I think it's more important to use the proper oil and change it at the proper intervals than what brand is being used.

BITOG has alot of discussion on this. Check it out.

Would you mix brands in an oil change?

I would not. It is possible that an additive clash problem could exist when doing that. Having said that I have done it on previous cars when I had a quart of this or that left over, the cars ran just fine.