Hello Lukas,
your inquiry is valid. When sizing your components and assigning tolerances, the primary consideration is to ensure the functionality of your parts. For this, the Geometric Dimensioning and Tolerancing (GD&T) method is the most recognized for implementing functionality-based tolerances, such as flatness, coaxiality, parallelism, positioning, and more.

You can access more detailed information on the Xometry.pro platform, in many articles, like:

Standard Tolerances in Manufacturing: ISO 2768, ISO 286, and GD&T

eBook: Mastering Tolerances for Machined Parts

However, your shop supervisor is correct: over-tolerancing is frowned upon and can significantly inflate your costs without enhancing functionality or quality. It’s advisable to use the standard general tolerances, as outlined by ISO 2768. This standard sets the tolerances for linear dimensions, radii and chamfers, angular dimensions, and also form and position tolerances: straightness, flatness, perpendicularity, symmetry, runout. And for all of these, with ranges depending on the dimensions. For what is not covered by these general tolerances, you can then apply fitting tolerances (ISO 286) or GD&T tolerances.

I hope this information is helpful.
Have a wonderful day!
Pierre-Yves

For details, please refer to：https://zscncparts.com/