The answer for commercial seed mixes is kind of "good luck" since each variety has its own preferences. Do the store-bought mixes have any advice on the seed packets? That's a starting point for each mix. (I think that when I have planted mixes, some of the seeds stay dormant for a year or two, then give me surprise "volunteers" for 2-3 years.)
With saved seed, each species has its own preferred time of sowing. Big seeds with hard coats probably need scratching and soaking, whereas dust-like seeds are their own pain-in-the-butt. It does seem too late this year to sow annuals in Iowa Zone 5. What's your average first frost date, late September? Save annuals for next Spring.
Maybe "spring-winter-sow" SOME annuals as a low-effort way to establish some seedlings without insect pressure or fussing to keep soil evenly moist for weeks. Start them in plastic, covered flats in some shade, before the last frost. They'll germinate when the sheltered tub reaches a suitable temperature, earlier than the soil in a raised bed would have started them. The covered tubs will protect them from wind, rain, insects and slugs. When they sprout, pop the lid and start to harden them off. When they're big, plant them out in chunks.
Or, if you have LOTS of saved seed and didn't give most of it away, you can simply broadcast annual seeds thickly in spring and rely on vast numbers of seeds to overcome less-than-ideal germination conditions.
When in doubt about the best way to sow mixed perennials, split up your stash. Whatever the best method is for each seed type, sowing 1/4 of your stash that way will probably produce as many of that plant as you need. You can afford to "experiment" with (waste) the other 3/4 of your seed. It may surprise you to find some easier or more productive methods if you experiment.
- Sow 1/4 now and hope they can establish themselves before frost.
- Winter-sow 1/4 of the perennials seeds in covered tubs or jugs, especially any varieties that you want to be sure you get lots of. Consider looking up each species that you saved seed from. If that seed germinates faster, easier or more completely with cold-wet stratification, winter-sowing is the easiest way to get high germination rates.
- Sow 1/4 later, like early spring, (whenever experienced gardeners suggest you have good odds for a "mix" that might include perennials, annuals and biennials, as many mixes do).
- Save 1/4 of the seed for the year after and for trades. If you keep track of which mix was sown in which spot, you can take a photo and say "this mix produced these flowers in the first year". Also, this year will show you the which are good ways to sow each variety or mix. Next year you can have the same success rate with 1/3rd the effort.
- Some seeds like poppies can be surface-scattered just before snow or even on top of snow! If you have poppy seeds or a poppy-rich-mix, give them what they like.
- big, hard seeds need a pre-soak and probably benefit from scratching. You might even sieve them out of mixes so you can give them a head start!
- Any saved seeds from varieties that you especially like are probably worth some extra effort. Set aside as many as half of those favorite seeds and look up the recommended method for each species. For the next few years, start at least a few of each favorite variety in the ways that are recommended most for that species. If they are perennial or reseed well where you live, you only need to succeed for one year to have a permanent patch. And now you know how to re-start that patch from saved seeds.