So, if I have enough yeast cells do I really need to make a starter?
Basically I have been saving the yeast slurry from the bottom of my fermenter after a brew with the intention of using it some time in the future (storing in fridge). I also have a vial of yeast that I grew from a bottle (one of my own) and poured into a vial after stepping up to 1 litre.
Does this mean I already have enough yeast or do I still need to 'activate' it a couple of days before hand in a starter?
The main reason is that sometimes I don't get to plan days in advance whether I can brew or not and it is usually a decision the day before which means I don't have time to make a starter.
The less prep I need to do the better

If I am using dry yeast then I don't need to worry about this, but since I am using liquid yeast more often then I want to be sure (especially after the longest lag I have ever experienced on my last brew!)