Please help w/ tipping seamstress

So I'm heading to RK Bridal in about an hour & 1/2 to pick up my wedding dress. I had 3 fittings and I'm done.

I've had 2 different women work on the dress because the one who worked on me the first 2 times no longer works there.

They asked me to bring cash & I'm wondering if I'm supposed to tip the seamstress. If so, how much is fair if the total cost of alterations is $350.

I wasnt going to tip mine since my alterations are going to be about $375