Yes you need to add seat reservations on some high-speed trains whether in 1st or 2nd class.

Usually it's the same price in both classes, for example French TGVs -> limited 10€, then 20€ until the train is full (much more often 10€ in 1st).

The cross-border TGV into Switzerland cost 70€ in 1st class which is way too expensive ! For this journey take any TGV between Paris and Strasbourg or Mulhouse (10-20€, see above) and then a comfy regional train across the border to Basel. From Basel loads of trains to everywhere.

For optional reservations (+ mandatory ones in Italy) use tickets.oebb.at (no booking fee) : FREE with a 1st class pass.

There are places to avoid booking fees, see :

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

https://www.seat61.com/interrail-and-eurail-reservations.htm

You pay 1st class for the quietness, bigger seats and more room + sometimes free meal (mainly in the UK)

About 90 % of all trains included in the Eurail offer have optional or no reservation so in most countries you can travel without reservations on many routes.

However,  during the summer it might be wise to make reservations on trains with optional reservation to ensure that you get a seat.

Night trains and high speed trains (not in Germany) have mandatory reservations and so have most trains in Spain. 

Where will you be travelling?