Shading your greenhouse will control the temperature if the ventilation system is not doing the trick. It also helps to protect vulnerable plants from direct sun. Shading also helps reduce scorching leafs and prevents flower crops from fading. If shading is being added primarily to control temperature, it should be placed on the outside of the greenhouse since inside shadings are not likely to reduce the temperature enough.

It does depend on the season and the type of plants being grown as to the amount of shade required. When there is strong sunlight, use shading that will reduce light by 40-50% for a typical mixed greenhouse. Ferns require even more shading, about 75% protection from the light, and cacti and succulents require very little, if any at all.
Greenhouse Shade
There are several different types of shading available. These consist of shading washes, blinds, meshes and fabrics and rigid sheets. With shading washes this is the most effective and one of the cheapest ways to reduce heat from the sun. It also helps to protect vulnerable plants from direct sun. Shading also helps reduce scorching leafs and burning of flower crops. To apply a shading wash, paint or spray the wash onto the outside of the greenhouse when the sunny season starts and then remove the wash late in the summer season by rubbing or rinsing it off with a cleaning solution. Washes are inexpensive however, can be a bit messy when applying and removing them. If the greenhouse is the main focal point in your yard, the washes can be a bit unattractive so this is something to be considered.

Another option for shading is blinds. When using blinds they are usually used on the outside of the greenhouse and control temperature quite well. The blinds should be very sturdy. These are more versatile than the shading washes since they can be rolled up or let down to give a specific amount of light. They can also be used in a specific area of the greenhouse if desired so you have more control over the light the plants will receive.

Flexible mesh shading material are used on the inside or outside of the greenhouse. The material is less adaptable than blinds because of being in a fixed position for the entire season. They can be less reliable at controlling plant growth than the shading washes. Woven and knitted materials are generally used on the inside and the outside of the greenhouse. The amount of light reduction varies a bit depending on the type of fabric used. Using the fabrics are not as reliable with temperature reduction as the other options are. Cross laminated fabric is a great alternative for interior shading purposes only. It can decrease the light by as much as 50% however does not control or decrease the temperature much.

Its best to make your decision as to whether or not you should shade your greenhouse based on the type of plants you are growing as well as where you chose to place the green house. Re-positioning the greenhouse is easy enough and can also correct the issue.