I lived in Paris for three years and did the following
- restaurants: usually no need to tip (esp if paying by credit card) - If paying by cash, 2-5 euros max
- taxis: usually add 5% and/or round up to nearest euro
- porters etc - a couple of euros

Certainly nothing like in the States; don't forget waiters, porters etc are paid the legal minimum salary and don't rely so much on tips as their US equivalents do.