For best quality Scale the texture via texture node the input vector. Use a Combine RGB node to send the correct values. Assuming that the width =1.0. Divide the height by the width for the width factor. e.g. (1080/1920), this will reset the aspect. Use a color curve to scale the vector.
To save you having to type in the current scene dimensions you can set them as a driver in a Value node.
First, here is the wrong aspect which Blender simply "fits" to the current aspect, in this example the scene is 16:9 while the texture is 1:1.
Add 2 Value nodes
Right click in each node value window and select Add Driver > single from target.
Use the proper to choose the scene dimensions.
Add a Math Node and set it to divide, then divide the width by the height.
The X aspect is sent to the vector input via the green register, correcting the distortion of the texture.
You can add another Math node set to multiply and a Value node to scale the texture after correction.
To re-use the aspect correction you can collect them into a Group Node. Beware that the drivers (in groups) fail if you change dimensions of the scene. If you need them to be dynamic then I suggest just copy and pasting the collected set of nodes, excluding the texture node of course.
Select the Value, Math and Combine RGB nodes and press Ctrl G to set them as a group.
Press the Tab key to step out of the group node and label it so that you can locate it later.
To add the group elsewhere in your nodes, Add a node and choose node group > (your node group name)
Attach it to your texture.
Remember that drivers will fail inside a group node, so they cannot change dynamically.