When it comes to warband maximum sizes, I always make mine slightly bigger tahn the upper limit so that you have options.
For example; My Marienburgers number some twenty models.
Although the maximum size of a warband is fifteen, it is good to have the option to field different combinations. You may think that you have everything covered but there may come a time when you would like to field something different. For example, at the begining of the warbands life you may feel that having one group armed with swords and shields is the way to go, but as casualties take their toll you may prefer to have a few warriors armed with spears so that they can strike first, even when charged.
It is good to have all of these things done before you start to paint the band, as it is easier to paint all the models at the same time, going back to add them later may not be so appealing when you have already moved on to painting something else.
In regards to Hired Swords, as previously mentioned, they eat away at your available cash pretty quickly.
I find that it is usually best to get the warband up to maximum size first and then add them, as this way their cost does not prevent you buying more warriors for your band.
Of course this does not always work. In an earlier campaign I had already built a warband up to maximum size, so I added a Dwarf Trollslayer and a Bard. Some games later I suffered four Deaths to the warband in the same game. (Terrible luck, but it can happen!)
As I had two Hired swords and four gaps in the roster, I found that the Upkeep costs to keep the Hired Swords was prohibitive to hiring new members and had to get rid of the Hired Swords, if I was ever going to get the numbers up again!
Hired Swords are helpful, but can prevent the growth of your warband. Also, as your warband gains experience, the difference between a HS and an experienced warrior becomes much less, and the HS does not look so appealing.
By all means Hire if you think it will be beneficial, but do not let them stall the development of your own band's warriors.
Having said all that, I have several painted models of HS, just so that I have the option to Hire should I wish to.
And to echo tomogui; Slim the collection? Never!
You will probably regret it later.