Use whichever representation helps you calculate accurately and communicate clearly. There is no advantage to forcing fractions if decimals are more natural for you.

The interviewer is primarily looking at your setup, accuracy, speed, and ability to explain the result. The notation is just a tool. Switching formats unnecessarily can actually create extra mistakes.

If 25% of 80 million is easier for you as 0.25 × 80 than 1/4 × 80, use the decimal. In another calculation, recognizing 1/3 may be faster than repeatedly working with 0.333.

Practice moving comfortably between percentages, decimals, and common fractions so you can choose the easiest representation for each calculation instead of following one fixed method. If useful, this guide on case interview mental math gives more detail.

Decimals are way easier for me too. I only switch if the fraction is obvious.

I don't think they care about notation as long as the math is clean.