I would disagree with Michael:
If you're building a shed or a house, you want to make sure both are structurally sound. So you would use the same methods, just at different scales.
Thats been my experience with utilizing UML: if you're building a small app or a large app, you're still ensuring that the base design is sound and solid.
Adding a methodology like Iconix or the RUP enhances this, by using techniques to ensure your code will be top notch.
Also, you should think long term about your web application:
right now we're working on a member portal. There's security, the portal itself, and hte business logic of the business using it. They also want a sales system, a purchasing system, and an accounting system inter-twined, some of that being web based, some being desktop based, all using the same database. Architecture is PARAMOUNT in this situation.
mit99: i'm not sure how large your web app is, but from my experience, utilizing UML and a software methodology can benefit you no matter what the scope is. Its more about getting into the mindset of how to develop great software than it is whether you'll "fully benefit" from it. My last company started using it, then stopped, and they still are plagued with horribly buggy code, long hours, and major design flaws that could have been avoided if they would have done full analysis and design first.
just mho
D'Arcy