I've owned a number of different TLR's in my time. Dust, haze, fungus, coating disintegration,any of these can be present in the taking lens. minor dust should not be a problem, but if it is excessive and there is haze/fungus present, then it will need a clean, otherwise pictures lacking in contrast will result.
An easy way to test the lens, is open the back, open the aperture fully, then hold the camera up to the light, and press the shutter on "B" holding it open. any dust, fungus, coating damage will become apparent.
A professional clean is he only remedy. If there is fungus coupled with coating deterioation it may not be worth the cost of a clean. Only you can decide.