You should generally tweak your technique in response to the target at which you are aiming.
The greatest force upon impact will typically be achieved by striking perpendicular to the strike zone, which is why different pads are often held in positions which facilitate this.
But of course there is nothing to prevent Muay Thai pads being held vertically and high. The consequence of this however will be that the striker's leg will be more prone to travel up along and off the pad (unless the striker is very flexible/mobile and can achieve horizontal impact at height). This slippage can also endanger the holder of the pads. They can also be held at an upwards angle to accommodate overhead punches, axe-kicks, crescent kicks and high, downwards roundhouses.
Similarly, there is also nothing to prevent a kick shield from being held at an upwards angle (such as on the thigh above a bent knee) which is in most circumstances safer and accommodates downward roundhouses well, ie: it provides a perpendicular target. A kick shield can also be held at downwards angles to allow for upwards front kicks and roundhouses, and vertically to cater to strikes which travel in the horizontal plane.
Your training will ideally replicate your combat strike techniques as closely as possible (an exception being when you are training purely for anaerobic/aerobic endurance). Given that in a fight, you will routinely land upon parts of your opponent's body which are not ideally positioned, your pad work should reflect this.
Remember too that pad work often ignores the 'offensive defence' of knee-checks and elbow drops, which can quickly rob you of - or dissuade you from using - your go-to techniques. This is why training without pads is also essential; so that you become accustomed to negotiating those parts of your opponent's body which can inflict harm.
In short, the pads you are using should not dictate technique. You should adapt your pad use to the techniques you wish to train.
