Well, mostly organic matter is best after it's been composted. But I think that most of your plants are probably acid lovers so top dressing with coffee grounds is probably okay. I'd also take some of it and compost them to mix in the soil. Just my humble opinion.

Would it be safe to add them to the plantiong hole when planting out new trees?

I always put coffee grounds straight into the soil. Every time I plant a rose, I put coffee grounds in the hole, plant the rose, then after 2 weeks when I fill in the lil basin around the plant, I fill it with straight coffee grounds. My roses have never complained. My mother always has done this too, and has always had happy, beautiful roses.
As a matter of fact, I just finished digging up my dahlias, cannas and hollyhocks on Friday, and so all the dirt in that bed is overturned..Today I went out there and threw a whole bunch of coffee grounds and this summers potting soil from containers right on top, and raked it all thru to mix.
I don't think there would be a prob adding them to a tree hole..maybe you might wanna mix it up 1/2 and 1/2 with existing soil from the hole, then put it in. Too much of anything is never good.
Good Luck,
Daniella

Thanks. Im planning on planting out some new gardenias and thought putting coffee in the bottom of the planting hole might keep them a bit more moist and improve the soil.

Adam