There is nothing to prevent you from pressurising the hydraulic systems for the walka round as long as you return them to the state prescribed by your company for start up. The hydraulic quantity gauges should give you any indication of loss of hydraulic fluid that you need to know as well if not better than a pressurised system during the walk around. As long as the gear bay is kept clean enough you should be able to notice where a leak has occurred by dirt sticking to the leaked fluid.
My advice is that you stick to the guidance or SOP that your company uses, but that you raise the question with your tech manager or chief pilot.