Round when it materially simplifies the calculation and the case does not require high precision. The goal is to preserve the business conclusion while reducing unnecessary arithmetic.

State the rounding clearly. If 49.8 becomes 50, say so, and keep track of whether you are slightly over- or under-estimating.

Do not round every number automatically. When the difference could affect the recommendation or when the interviewer gives exact data for a reason, keep more precision.

I usually ask myself whether the rounded answer could change the conclusion. If not, I round.

Saying the rounding out loud makes it look intentional instead of careless.