Wojtek is dead right on all points, and that's the way I do things too. OK, paint pigment settles, but it has also 'weakened' the thinners, leaving it, at least after second use, only good for cleaning paint off your hands, or the outside of the airbrush or other equipment. It only takes a very small amount of paint residue to contaminate the next batch, which can not only give colour problems, but can lead to chemical reaction with the paints after spraying, due to differences in 'strengths' of the thinner. Although I agree that acrylic thinners such as the one supplied by Tamiya are horrendously expensive, isopropyl alchohol is not, relatively speaking, and that, basically, is what Tamiya thinners is!
If you use enamels, then a 2 or 5 litre bottle of White Spirit costs very little, in comparison to a very small bottle of the same stuff branded as a dedicated (paint manufacturer's) product. So, apart from re-cycling for secondary cleaning, it's not woth re-using thinners for thinning, or cleaning the internal components of an airbrush; not for the delicate requirements of modelling anyway - it will lead to accumulation of pigment deposits, and these will be more corrosive (on the parts) than 'fresh' thinners, resulting in a shortened working life of the tool, and increasing operating problems as time goes on.
In photography, when it was all film, the film was the cheapest single item in the overall process, so it was always the norm to shoot off rolls of film, knowing that at least one or two shots would be 'the' ones, rather than shoot only half a roll and miss the 'dream shot'. It's the same with thinners - it's the cheapest item, overall, in modelling, so don't b*gg*r about trying to save half a cent at the expense of a good result - dump the bl**dy stuff!! (In an 'environmentally friendly' way of course!).