Good question, debodun. Here's the thing….the cash register, at the end of the night, will not balance if you don't point out the 30 cent error and the boss will know the server messed up. You're doing the server a favor to point it out to her so she'll have time to correct it before the night audit drawer count.
I use to do the audit for a hotel. When the draw count is off, even if it's a penny, it's an issue. I had a "kitty" of change for errors under a dollar and never made mention of it to the boss but still the boss may notice and if the mistakes continue, the server could be let go.
The cash drawer count and the tips are separate altogether, I assume (I didn't do an audit for a restaurant)