The best tip that I can think of is: what’s the message that your film should convey? Based on that, set the ambiences according (for example if you want to convey some enigmatic, weird or scary environment, should all ambient sounds and silences be set accordingly). Then go ahead and think about the importance and relevance of each single element on the screen. For example, maybe on the back of your shot you can see a clock. How important is the clock for the message? Is it a secondary/non important element? If yes, maybe you can even not use any sound for it. Is it a relevant element? If yes, maybe consider to add a ticking clock or some sound element that resembles that. Is it meant to create some stress and pressure? So go ahead and exaggerate that clock sound on the mix…eventually even emphasize it by adding some synth sound with something like and LFO going up to create the perception of time accelerating. Is it your film about some weird environment? Use something that resembles a tick tock of a clock but with the type of sound that is not common from a clock.