The importance of where your Greenhouse is in your garden depends very much on what you want to grow in it and here are my thoughts on growing and how you can place your greenhouse to maximise growth of early crops.
If you want to grow crops all year round then for most of us it involves either a greenhouse or polytunnel and of these a greenhouse is better at keeping warmth in so how would you maximise the available light as this is vitally important with shorter days and far less intensity of available light.
It’s best to line up the ridge of the structure to run east-west as this faces the Winter Sun this will maximise light during the winter and it will also help it to heat up more quickly after cold nights. In fact if you orientate some of your vegetable beds this way it can make a marked difference and it was often used to obtain early crops in some walled gardens by Victorian and Edwardian gardeners.
One of the best examples of this is at the National Trust Trengwainton gardens were their walled garden houses some unique growing beds. After its initial construction in around 1820 (built to the dimensions of Noah’s arc incidently) the Walled Garden was divided by sloping beds, angled towards the winter sun which aided plant growth and drainage so the soil warmed up more quickly a technique we now use in raised beds. The beds were constructed using brick although there are no clay deposits in Cornwall and the raw materials had to be shipped from Somerset as they retain heat much better than stone.
If you only intend to grow summer crops, aligning the ridge north-south is preferable as it gives an equal amount of sun to each side and helps to reduce overheating on the hottest days but this would seem a waste of your greenhouse for quite a few months of the year when you could grow cold hardy varieties of many vegetables.
You may also be overshadowed on one side of you garden or perhaps have a high wall or hedge to a neighboring property in this case making an uneven span greenhouse were the maximum area of glass faces maximum light, these can look very attractive and work extremely well as growing spaces.