Paul
Just being grass doesn't mandate a performance take off, IMHO.
What might make the difference when comparing hard and grass is the effect of the factors applied to wet, long (etc) grass, leading one to operate that way.
I flew regularly out of a 700m grass runway and I would run up the engine against the brakes to 2000rpm to see the T&Ps stabilise and use flaps 25 to shorten the ground roll, flying off at around 55, instead of 65. As there were no obstacles ahead, I would trade early lift off for poorer initial climb out.
In principle, on a 700m hard runway, I'd have used flap 0.
The law of the situation applies and I don't think that you are being over cautious, you are still a relatively new PPL taking a conservative view and thinking about what you are doing - that's good airmanship in my view.
Have you seen Safety Sense 7 leaflet form the CAA?
http://www.caa.co.uk/docs/33/ga_srg_07webSSL07.pdf
The performance factors in this book are felt by some experienced pilots (I am not in this experienced category) to be ultra conservative, but they make a good starting point when considering take off and landing performance.