Always try to keep 10-15% in reserve for "while we're at it's" and "I never thought of that's". There are a lot of little costs with a new home that are not included in the builders price. If you are able to draw on the construction loan to cover those costs, you'd be able to use the money. If not, then you are just paying interest on money you won't use.
The fixtures can be an expensive item. I presume that he's giving you an allowance. When it comes time to shop, don't treat that allowance as free money. Remember, that "it's just a little bit more" will add up to be $10,000 in overages. Often these builder like to price entry level fixtures (moen, price pfister, etc) to make their bid look good. There another places where overages can easily occur. Landscaping, well, septic, utilities, finishes, exterior painting, roofing, windows, etc. Hopefully you made all these selections before signing the contract so you have a dead on price. If not, then you can count on some more overages.