Are you supposed to round in SUVAT equations (OCR A-level Physics)?

I was always taught to never round until the end; however often my teacher and online worked examples will round in the middle of the equation. For example, in SUVAT equations they question might be to find T and they need to first find S to do this, and they'll use S rounded to 3s.f rather than the exact value in their equation to find T, which gives a different final answer. Further, they'll use a rounded version of the V value (eg, instead of 100cos45 they'll use 70.71)

Is this a valid method, or should you always use the exact value (if it has a lot of figures, store it in your calculator)? Or you actually supposed to round early like this, and I'll lose marks if I don't?