Do I need to incorporate in the US to offer SaaS?

The basic rule of Sale of Goods internationally are the Vienna Convention with Sale of Services mostly determined by where those services are performed/cosumed. Most tax authorities are interested the real economic activity with various rules that apply, depending on:
whether you have more than 1 business location
the kind of service you provide
the place where your business or your business customer ‘belongs’

However, the general rule is if you supply services to a: business customer the location is by default the place where the customer belongs and for non-business, where the supplier is. Unless Brazil has special exceptions for certain sectors, there should be no need to incorporate in US as international contracts that clearly spell our jurisdictional issues are part and parcel of modern world.