we will be in Paris for 5 days in June. do you recommend we purchase our train tickets to amsterdam from the US, before we go?
Yes, to take advantage of discounts that may be available. However, that does lock you into a specific schedule.
Where you are doesn't matter, but when does. The sooner you book your tickets the more likely you are to get cheap fares.
Book at: https://www.thalys.com which is the company running the trains between Paris and Amsterdam.
You can book 3 months ahead, so currently bookings are open to 1st June. Prices for 1st Jun are now at €35, prices for tomorrow are €132, that is a big saving for booking in advance! But, when you book you must commit to a date and time, you cannot later change your reservation.