Done wrongly, your Conversion Tracking can get you lost and cost your business dearly. Worse than being inaccurate, wrongly implemented or wrongly interpreted conversion tracking results can put you in the position where you are pulling out marketing funds/efforts from "seemingly unproductive" acquisition source and wasting the funds on the wrong channels.
Depending on your business nature, common purchases may be made by your customers within the first visit or after a few subsequent visits. The latter is particularly applicable on high value items, requiring more thorough research and conscious decision making by the purchasers.
For purchases made through multiple visits, your customers may be visiting your website (and other websites during their research) through multiple channels, including searches (paid and organic) for generic keywords, clicks on ads, referal from other sites such as comparison sites, news, email (some call this Spam :) and even directly entering your website address into their browser.
In this, it is important to make up your mind on which source to actually attribute the sales to (i.e., campaign attribution)