For customers where you expect to make recurring revenue through a support contract, it can be a very powerful bargaining chip to take a cut on profit margin for the initial installation. That is very common in my industry, which is networking for hotels, but we don’t do it for every job. We’re a large enough company that we have separate sales persons, and its their discretion when and how much of a discount to offer.
For your position of using a discount on a one-time job to curry favor for future one-time jobs, the situation is more difficult. I have heard before that if the potential customer asks for this, as in they imply but don’t outright promise that a discount now will get you more work in the future, to try to stay away from that customer - they are not business you want, unless you are desperate for the income. But if the customer doesn’t ask for a discount, and you are comfortable with them, I don’t see any issue in offering a discount. Just make sure its clear in your line-item invoice - put the discount after the subtotal but before the taxes.