There's an old saying/reality in the software business:
"Customization is the bane of software companies".
That's because every customization you make to an existing software platform causes one of two things:
1. It causes you to write custom code that may cause you to run different versions of the software to accommodate the changes a customer is requesting. You have to keep up with different versions. OR:
2. Customization in your core single product that everyone isn't going to use just creates complexity and layers to the code bases that makes future releases/upgrades/bug fixes more problematic. The more custom code you write in a single product, the more things will break or need band-aided in the future.
The bottom line is that customization causes complexity. The same logic holds true for your HR shop. If you're good, you've got a set way of doing things, and if you do it the same way often enough, it's going to work pretty well. But you'll have requests from your client group often to do it different ways. It's hard to say no, but you should say no when you can. Complexity eats away at your ability to deliver in an efficient way.
You know when customization for your HR client group really makes sense? The same time that it makes sense for a software company. When the work that you'll do to customize creates features that can be rolled out to more than one person/client.
Say yes to custom work that results in your HR practice being deeper and capable of delivering more. Make sure you approach it like a product manager, to make it replicable.
Run away from other custom work if you can. But the take above means that if you run away every time custom work is requested, you're probably transactional - not strategic.