Porters going thru customs: $5 per bag. Engage them before checking bags at airline counter as they cannot accommodate you if already in customs line. If line is long(ie coming outside the double doors), certainly worthwhile.

I never have used porters at baggage claim.

Dock hands/ fuel is $5-20 for me.

Briefer is $20 or so.

Restaurants are similar to USA for me. 20% (and often 25%). If gratuity already added (ie service fee), I will add another 5-10% especially if they point out fee is added (or clearly listed). If sneaked in, I usually will not added additional.