Seed Compost

If you are sowing into individual pots / modules, and you are sowing things that germinate easily, then it probably makes little odds

If sowing in a seed tray to then prick out I only ever use John Innes Seed Compost (and a good brand at that). In a season I only use a single small bag (and I sow a lot of seeds), so the cost is not a significant part of the overall season budget.

But I do reuse the seed compost for successive batches, within reason:
1) Nothing diseased is reused of course, although such disease is, in practice, almost never
and 2) I have to be aware that late germinating seeds that come up in successive batches are not mistaken for being the desired plants themselves!

A decent J.I. Seed Compost is very [sandy] fine. It just falls off the seedlings during pricking out. Whereas anything with lumps of peat will mean that multiple seedlings all have their roots into a single lump, and these get broken when pricking out.

Again, varieties that grow like weeds won't be too troubled by this, but things that are more "challenging" may be.

I also think I get better germination rates. Again, if you are sowing Lettuce and get 20,000 seeds in a packet its a non-issue, but for things which only have half-a-dozen seeds in the packet, particularly for a "troublesome variety", then I think it makes a difference.