Distillation of water will not remove all hydrocarbons or chromium-6, without very sophisticated equipment. It can remove arsenic, salt, and other such nasty compounds. Source:
5 Things To Know About Chromium-6 In Drinking Water
In practice, large-scale water purification systems expect water to contain bacteria, parasites, viruses, sediments, chemicals, and VOC. In a typical setup, such as NYC, which processes 1.4 billion gallons a day, and which takes appx 8 hours for water to enter into, and exit, the filtration systems, rest assured that distillation is not a part of this process.
You can read about how NYC handles its water, and if you are so inclined, you can arrange for a visit to their facilities, which requires special precautions and arranging, and may not be suitable for everyone. Nevertheless, I worked there for several years and found the treatment processing fascinating. Here is the link:
New York City’s Wastewater Treatment System (1)
New York City's Wastewater Treatment System (2)
So you don't need (or want) distillation to remove the worst of the worst. Having said that, you can't just drink VOC-contaminated water, like you could drink disease-infected water, and just deal with the consequences later on. That's because the human body reacts violently to VOC-contaminated water. Here are some studies on the effects of drinking such contaminated water:
The Health Effects of Oil Contamination: A Compilation of Research
Since the bulk of questions here on this forum deal with personal outdoor issues, I'll relegate my answer to that kind of scenario.
There are many products on the market for treating water, and all involve either chemically treating the water, filtering the water, or just boiling the water - and none of them will remove VOC or chemical compounds like chromium and arsenic. Nor will they remove compounds which are solutions - like salt. VOCs are volatile organic compounds, like oil and benzene. If you are in a situation where you have to drink this water or go without, you'll have to decide that on your own, based on information such as nearest other water area, your tolerance, and other things.
But distillation is an impractical form of water purification for most individuals. The heat needed to distill it would kill off most common bacteria, virus, and parasites. What's left are the things which cannot easily be removed by distillation, leaving you with a conundrum best handled by not bothering to distill it at all. Unless you were dealing with salt water - then you have no choice but to distill. Just know that it wouldn't get rid of all VOCs.
I think, then, the best way to treat water without products like tablets and filtration systems, or distilling, is to make use of what is already in nature.
So you ask "when should you boil it before distillation", my answers is "all the time - but skip the distillation step". Since you mentioned a concern for petroleum (VOC) contaminant, you should use the soil - sand, if possible - to filter and absorb such contaminants. In other words, you think about mimicking what NYC does with their water, only on a smaller scale:
Remove large sediments
Let stand (that's what the big circular vats are doing to the water, only you don't have vats)
Siphon off water about 3/4 from the top - that removes water between the floating oils and the sunken solids. Do this without disturbing the water as much as you can.
Next up is to filter the water using sand - lots of it. This will absorb some of the VOCs present, though not all of it.
Last, you can boil water to kill off microorganisms.
NYC and other municipal systems go through steps like aeration, ozonation, and chlorination, and these are impractical methods for most individuals.
If you were truly concerned about removing VOC, then you must use aeration with activated carbon, or use a reverse osmosis filtration method:
VOCs: Volatile Organic Chemicals in Private Drinking Water Wells
Note also there are no purification methods for radioactive contaminants in water, and there are many sites in the US with this problem:
Defenders of the Black Hills - Hesapa O'nakijin
Radioactively Contaminated Sites
By the way, there are some great articles on purification from the US military, although I haven't found much on oil-contamination:
Small Wastewater Treatment Systems Using Soil Purification Method
and also from the CDC (also little on oil contamination):
A Guide to Drinking Water Treatment and Sanitation for Backcountry & Travel Use
Emergency Disinfection of Drinking Water