Well, you will have to decide that based on your own reaction to certain foods and food types. Typically combo bolus is used when your spike may be delayed (foods like pizza, chinese take out, high carb/high fat combinations are often culprits).
Many people also use it as a tool for grazing, say like when you are at a party and are nibbling a bit here and there.
My personal experience with combo bolus hasn't been all that successful, but I admit I have to do more experimenting with it. I do the lazy person's combo bolus and usually if I am in a situation where I need an extended bolus, I will just do a temp basal increase, which works better for me so far.
The best way to find out for yourself though is to test. Pizza is a good food to experiment with. Almost always it will give you a delayed spike 3-5 hours after eating. Test first using your regular bolus, and test every hour for 5 hours, and then the next time try a combo bolus over 5 hours and test every hour, and see how your readings compare.
(Me, personally, and I'm not suggesting you try this, but for pizza I usually have to take 150% of the bolus amount, with 100% up front and 50% over 4 hours. So if I'm eating 30g of carbs in pizza I combo bolus for 45g, with insulin for 30 g immediately and insulin for 15g spread over 4 hours. I sometimes also do a temp basal increase of 20% on top of that depending on how much I eat).