OL, 'fraid there isn't a one size fits all, when it comes to this subject. It will usually have letters on the seed packets that represent the hardiness of your chosen plants i.e. sweet peas will have HA printed on the packets.....this indicates they are 'hardy annuals'...but we know they are hardy, didn't we? ....others will say HHA, this translates to half hardy annuals, meaning they can be planted out after no more frosts are expected etc.
Tomatoes Cucumber Aubergines & Chillies are not normally hardened-off, because they are grown under glass for the duration. They can be grown outdoors, but this is something undertaken during the warmer days of summer.
Marigolds Lobelia & Morning Glory can be started under glass before being hardened-off by putting in a codframe for a couple of weeks before being planted into their permanent positions....leaving the top off during the day & replacing it at night.
Hostas & hardy geraniums no problem with those.