In many cases, the judgment call on this by the immigration inspectors is very subjective. It helps to remember that the immigration inspector only has two real concerns: 1) Does it appear that you might want to overstay your welcome?, and; 2) Does it appear that you might become a burden on the public purse while you are in the country?
If, for example, you are middle aged, married, and traveling on business, both of the above scenarios are unlikely. They don't even ask you. By the same token, if you walk out of the business class section of the plane, well dressed, carrying a Canadian, American, or Western European passport, they will never ask to see if you have a return ticket or sufficient funds.
On the other hand: If you carry a passport from a country that most people would give anything to escape from, and if it looks like you are carrying all your worldly possessions with you, and you are young enough that you don't appear to have any ties that bind you to your home country (wife, own a house, etc.), then expect the third degree. There are obvious economic and social incentives for you to never go home again.
I'm a Canadian passport holder, male, mid 40's, who travels internationally at least once a month. I've never been asked to show an air ticket or sufficient funds to an immigration inspector in my life. But younger co-workers of mine who have visited Canada on Algerian, Angolan, and Chinese passports have had a rough time even getting visas to come to Canada on business trips, when everything has been prepaid and guaranteed by a Fortune 100 international company.
To answer your question, pretend that you are an immigration inspector, and in order of importance, consider the following:
- What passport you hold
- What you look like when you get off the plane
- What luggage you are bringing with you
- If it appears you have solid reasons to return to your home country (wife, kids, job, property)
Now you make the decision: Wave yourself through, or question further?