Spread policies change a great deal from broker to broker, and the policies are often difficult to see through. This certainly makes comparing brokers much more difficult. Some brokers actually offer fixed spreads that are guaranteed to remain the same regardless of market liquidity. But since fixed spreads are traditionally higher than average variable spreads, you are paying an insurance premium during most of the trading day so that you can get protection from short-term volatility.
Other brokers offer traders variable spreads depending on market liquidity. Spreads are tighter when there is good market liquidity but they will widen as liquidity dries up. When it comes to choosing between fixed and variable rates, the choice depends on your individual trading pattern. If you trade primarily on news announcements that you hear, you may be better off with fixed spreads. But only if quality of execution is good.
Some brokers have different spreads for different clients based on their accounts. For example; those clients that have larger accounts or those who make larger trades may receive tighter spreads, while the clients that are referred by an introducing broker might receive wider spreads in order to cover the costs of the referral. Some offer the same spreads to everyone.
Problems can come up when you are trying to learn about a company's spread policy because this information, along with information on trade execution and order-book depth is rather difficult to get. Because of this, many traders get caught up in all of the promises they hear, and take a broker's words at face value. This can be dangerous. The only real way to find out is to try out various brokers or talk to those who have.