"when it comes to the assignment clause.... Is this something that is often frowned upon?"
It can be, it all really depends on the people you are dealing with. Even though the seller agrees to a price with you, if they see you make money assigning the contract, some people, as a seller, may not like that. They will see you as a middleman in the deal. Also some sellers will not care. They got the price they contracted with you and they are happy it closed, even if with someone else. It all depends on the type of person the seller is, as to how the seller will feel about the deal.
Also when the seller is a bank, or in probate, or with attorneys involved, or some Realtors, or any other traditional person in the deal, they might frown on the assignment. Example: they make a deal with a person who wants to assign it, and it never gets assigned and then never closes, the seller feels like they were given the run around because the deal fell through. And unfortunately, there are several investors who have not been able to assign the contract and didn't close the deal anyway. They used a weasel clause to get out of the contract and people frown on that. Many traditional sellers or pros don't want to take that chance and will frown on assignments.
With that being said, you should have a plan to close the deal if you can't assign it. If you use the weasel clause too many times you will get a reputation in your town as someone who can't close. Now with that being said, make sure you have a good deal to start with. Most newbies who try to wholesale miss out on many of the other expenses involved in buying and selling a house. What they think is a good deal, is really "not a good deal". That is the most common reason they can't wholesale it, and if its a marginal or thin deal, you may not want to close on it yourself. Remember the main objective of wholesaling, finding a good deal that other investors will drool over and want it! If other investors don't drool over it and want it, its probably not a good deal. Some wholesalers get greedy and don't leave enough room in the deal, but some just don't realize it at first.
Your next question "and so I have to make sure to have a contract with the buyer, dictating the when's and how's, prior to assigning the original seller's contract?"
Not really, when you assign the contract to a new buyer, the assignment itself will dictate the terms. The assignment is the agreement and it can be called an "assignment agreement" by some people and other names by others. But the assignment itself will say when you get the fee such as: all up front, some upfront and some at closing, or all at closing. It will also state what happens if the closing falls through, and what to do on "why it didn't close" such as the original sellers fault, the new buyer's (assigned to) fault, the title or properties fault, etc...
But really what you need is a good wholesale course and they are many available. Most courses will have the paperwork and most common scenarios that you will run into. I am not recommending any course per BP rules but even when you get a good course, you will still have many questions in the beginning so networking and BP is a great place to get your questions answered.
Think of it as college, there are ivy league universities, private universities, state universities, and even community colleges at the county or city level. Most all will give you a good education. Many people try the "Good Will Hunting" approach: [“You wasted $150,000 on an education you coulda got for $1.50 in late fees at the public library.” ? Matt Damon in Good Will Hunting] but what many people don't realize, he spent as many hours reading books and the students at the university spent in class.
OK, I'll stop rambling on...