Would it be very risky to not book accommodation for last week of our trip in South of Spain along the coast? We would like the freedom stay where ever we feel like at the time but don't want to end up paying for very expensive places either? Anyone any experience of doing this? We would be in areas cadiz and down all along the coast back up to malaga?
As a frequent traveler I don't see that as risky at all. You might not get your first choice of places that is all.
All you need is internet access and your preferred website for booking independent or hotels that are part of a chain.
I use booking.com on a tablet or phone with the ap that has my credit card registered on it so I can book stuff like you are trying to do on the go. Or I use Accor hotels ap which allows you to book the consistently the same services Ibis Hotels
Much of the stretch of coast near Marbella is extensively built up. I doubt you’d have any trouble. For the most part, the end of May is the beginning of tourist season, so prices may be a little higher than the would be the beginning of May.
It's always a trade-off. Book early, you get a better choice (location, amenities, price) - but you are "locked in". Wait and book on the fly - you will have more limited choices (location, amenities, price), but you will have more freedom to change your plans. OTOH, you will have to spend at least a little of your time (maybe more than a little? depends on how picky you are) looking for and deciding on your accommodations.
Personally, I've given up trying to "wing it" and although it may sound counter-intuitive, I have found that this actually provides me more freedom. I know right where I'm going, I don't have to spend any time searching the internet "shopping", trying to find the actual place (it's in my phone and GPS), weighing options and prices - I can enjoy a glass of wine and a plate of pasta as I watch the world go by. Different folks enjoy different styles. I'm OK with having my plans (at least for accommodations) nailed down before I start packing. YMMV.
I think winging it, when you can, makes a lot of sense when you're going to an area for the first time and don't really know how much time you'll want to stay in each place. I worry when people show up on the forum, booked into a coastal town for a week. Some of those towns are really not good destinations for the typical American tourist looking to have a Spanish experience.
I would definitely book in Cadiz. Will you be driving or using bus/train? Some of the towns are quite small and you could get stuck. Have a good plan in place like Jazz recommended (an app on your phone, a little bit of scoping out your options).
You can monitor the availability of lodging at each potential stop on booking.com. Check daily and you'll have an early warning if a town is getting booked up and becoming expensive. You can then either go ahead and make a reservation or decide to stay elsewhere. Do be careful about cancellation policies. They can vary.