Pointing SPF to Valimail is perfectly safe and it will not cause any deliverability issues if your domain is at DMARC policy p=None.
Pointing SPF to Valimail will not change the contents of the SPF record on the domain you perform the pointing for, this action just means 2 simple things:
You will be transferring management of the SPF from your DNS to the Valimail platform.
It will allow you to use Valimail's patented SPF Macro technology with the help of which you can have as many approved and configured services as you want for your DNS, without having to ever worry about the 10 SPK lookup limit anymore.