Should trainees top up our CPF since we don't receive CPF contributions? Or is it better to invest instead?

If you want it, yes you can. You even get to enjoy some tax relief along the way :)

My main reason for topping up CPF is not just the interest but also the significant income tax reliefs. Without the reliefs, I'm hesitant to lock up my cash.

If I were you, I would invest my spare cash. I will only consider topping up CPF when I reach a higher tax bracket and the income tax reliefs are "worth it".