For "pure" ASP.NET, there isn't really much of an issue because by design, the aspx page and the code-behind document are separated: the code-behind document follows the indentation rules of the language used (C#, VB.NET, ...), the aspx page follows HTML markup.
Apart from this, I always let every language have and keep its own "default" indentation. But let's be honest: this is about more than ASP.NET. It's about making consistent choices in your programming habits. It's not about writing code you can print out and frame. It's about consistency. Choose a set of rules within the range of established possibilities and stick with them.
Check out articles like Top 15+ Best Practices for Writing Super Readable Code. It's packed with good advice. Undoubtedly, there are many others, that can offer valuable insight as well.
(If you're in a work environment, team habits or team rules may apply: dear employee, please do as we do. That makes sense.)