For the jam, cooking to the setting point (105C) is essential otherwise there is a risk that the jam will be significantly under or overcooked. This means it may not set at all or be too stiff to spread. As far as the Italian buttercream goes, getting the sugar syrup to the right temperature (115C) is vital to not only render the egg whites safe to eat without baking but also to produce a stable, velvety-sticky meringue that can be used for making a particularly sumptuous buttercream. So do yourself a favour and pick up a reliable Thermapen digital food thermometer and don't look back!