When following hop IBU in recipe

When you are working on a recipe and it lists the IBU for a hop entry, do you alter the recipe to make sure you hit that IBU or do you just follow the guideline for amount? So far I've just ignored the IBU and followed the recommendations for weight but on the pale ale I'm about to brew I had to substitute for a different hop. So I'm wondering if I should just tweak the amount until I hit the IBU suggested in the recipe.

In general, if you're trying to follow the recipe as close as possible and need to substitute hops, yes you want to tweak the amount based on the AA% of the hops you are substituting and the AA% called for in the recipe. Which would accomplish your goal of hitting the IBU number.

However, there's nothing that says you can't change the recipe a little by substituting a different hop and not changing the amount. It's all up to you.

Edit: To clarify, you want the AAU's to be the same. AAU = AA% * oz. So if the recipe calls for 1 oz of 6% cascade, that's 6 AAU. If you want to substitute 12% columbus instead, the math is easy, you'd have 6 AAU / 12% = 0.5 oz. If you're substituting 9% centennial, you'd use 0.67 oz.

In my case, the recipe called for .66 oz Horizon at 13% which calculated to 34.2 IBU. I couldn't get Horizon so I got Magnum at 13.1%. It also called for .25 oz of Cascade at 6% but the Cascade I got was listed as 7.5%.

My assumption is that I need to somehow tweak the amounts in BeerSmith to match the resulting IBU amount.

As I said, just do the AAU calculation to adjust the amounts, BeerSmith will probably even do it for you but I've never used BeerSmith so I can't say for sure. I know BeerAlchemy (which I use) will do it.

*However*, with the numbers you just described, I wouldn't even bother. The differences are so minor that I'd just use .66 oz and .25 oz of the hops you have.