It all depends how obsessional you are and how much yield you want from your patch.
I have planned obsessively the past two years, but what you cannot plan for is the British weather! Spring harvests may be earlier or later in particular.
The more you know from experience what works in your garden, the better return you get on planning.
The first time you plan, the plan may not work in totality because some things did not work!
The best planning is around half-season crops because then you get two successsful half-season harvests.
The other good thing for planning is how much you need for a year: four 1.5m rows of parsnips last an entire winter! However, you may need an entire 5m * 1.5m bed of onions if you make lots of stock, cook italian and consume lots of soups. 250-500 onions will come from a bed like that.
The lesson I have learned from planning is when your plot suddenly becomes fertile and pest resistant as you crack the compost problem, you end with gluts as your previous yields are multiplied 3-4 fold! Radish, carrots and beetroots for me this year.