if you use a polytunnel or greenhouse it is important to ensure the plastic or glass is clean. Even glass cuts out 50% of light (apparently) so if it is covered in a layer of dirt then light levels are going to get really low and things will not grow so well.
Do it now, while there is not so much inside so that you can get to all the surfaces.
I clean mine with Ecover washing up liquid in warm water and use a soft brush just to scrub off the dirt and then rinse with the hosepipe. I need a short ladder to get to the top!
it isn’t until you have cleaned it you realise how dirty it was!