Depends. You just need to make sure you get paid the proper amount in the end. Some clients want hourly, some want fixed cost. In either case, you must invoice periodically, not all at the end.
If you work hourly, begin with an up-front retainer. If you bid fixed cost, get 50% or 25% or whatever up front. Don't start coding until you get a check.
Once someone has paid up front, the chances of them paying in the future go way up.
What about situations where a client wants you to "take a look at the code"? In my very very limited experience I've told them I'll spend an hour or two on spec going over the source to evaluate before giving an estimate. Is this too little, in your opinion?
Do whatever you have to do to get the sale, but realize it's on your dime. Once you have agreed to a contract, get paid and then start.
I usually meet with the client, look over their wireframes, and do a little research before signing a contract. Typically, that takes me an hour or two. Any more than that, I usually want to start getting paid. But do whatever works for you.
If you work hourly, begin with an up-front retainer. If you bid fixed cost, get 50% or 25% or whatever up front. Don't start coding until you get a check.
Once someone has paid up front, the chances of them paying in the future go way up.