This might not be what you want to hear, but in all honesty, I don't use any on most ornamentals. It's a lot easier to cause harm (lots of leaves at the expense of blooms, or too much of a non-soluble mineral building up in the soil), so as long as my plants look happy, I don't use any commercial fertilizers. Scott's and other similar companies are in the business of selling you things, not necessarily providing what plants need or benefit from most. Also, one of the nonpoint sources of water pollution is the fertilizers added to yards and farm fields which mostly dissolve quickly and anything not immediately used will wash away with the next rain. Having a good organic content to your soil will help prevent this since the fertilizer components are likely to cling to the organic particles in soil.

I do turn compost into an entire bed when I am first developing it, and I top dress with compost and then wood shavings as mulch. Over time these materials slowly break down to provide the nutrients that the plants need. If you are concerned, have a soil test done to see if there is something lacking, but even on my relatively young soils (I live in an area that was a lake bed during a period of glaciation, so my soils tend to be outwash from streams that fed the lake, largely sand and gravel) I haven't found that most plants need anything more than compost and organic mulch. I do have a bag of alfalfa meal that I bought to use on clematis and roses as needed (still sealed after 2 years), and something like the Espoma line of fertilizers are designed to break down slowly and so are unlikely to cause harm if used according to directions.

The main exception is for plants growing in pots which don't have the soil critters to help break down organic matter and so rely on what you provide. They can have pelletized slow release fertilizer added at the time of planting or you can use a really dilute solution of a regular soluble fertilizer when you water. I also sometimes turn aged manure into my veggie gardens before planting since veggies tend to be heavy feeders.

Other special cases would be if you live in a new development where the topsoil was stripped leaving only subsoil or live in an area with similar soils to mine such as parts of Cape Cod. However, in that case I would be more likely to thoroughly prep the beds with lots of compost or aged manure turned in to bring up the organic matter in the soil so it more closely resembles the organic content of typical topsoil. The first year until the organic matter starts breaking down, you might need to add a fertilizer to help plants get started.