For the best practices, I always think about to create a follow up guideline or checklist before creating any Form and Workflow.
For Nintex Form, which Form approach are we going to use, Responsive one or Classic one and to list down all the capable features and non-capable features to differentiate between the two approaches. Therefore, it is easy to identify the requirements and potential issues for each approach.
For Nintex Workflow, my opinion is to identify which logical flow are we going to use, Sequential or Stage Machine. There is pros and cons for each one.
Also how we are going to utilize the workflow history and task list. Are we going to use each workflow for each task list and workflow history or using one task list and history for all the workflows. There is performance issue we have to think about with number of workflows and instances that will occur in the future. Another thing to consider to place all the workflows in one site collection or not. The advantage to place all the workflows in one site collection is centralization and the approver can delegate his/her tasks to another user easily during his/her vacation through Nintex Task Delegation feature.
I'd like to hear the opinions and best practices from others members as well. Appreciate for your contribution. Thanks.