Hi Karen, if on your date of departure, you have not exceeded 90 days in Turkey in the previous 180 then you will be fine. However, this criteria will have to be met each time you depart Turkey.
For peace of mind it is probably better to take out residency. Of course there are new rules regarding residency coming in April 2014. It is intimated at the moment (and as this is Turkey, so it could change), first application for residency will have to be made to the Turkish Embassy in London - incurring a fair amount of expense unless you live in the capital. The other thing is residency will only be granted for a year at a time.
If these plans come to fruition, it will be a lot more difficult and more expensive to get residency, especially the first time.
If you will be in Turkey before April 2014, it may well be better, if at all possible, to take out residency before April 2014.