I wouldn't say they are a must if you are sticking to a UK legal 250W motor but you should take more care with Aluminium forks. What model fork are you using, or any chance of some pics?
If you are going to use aluminium forks there are several things that you should consider. Firstly they're not as forgiving as Steel, so if they go, they can go in a rather catastrophic way
You don't want to loose your front wheel, even going at the UK legal limit of 15mph as it's fast enough to be kissing tarmac.
You should be addressing 2 points when fitting a hub motor to front forks. Firstly aluminium suspension fork are generally designed with quick release rims in mind and so therefore they have what is referred to a "lawyer lips" by some, but basically a depressed section where the quick release fastening sits into. Secondly standard bicycle forks were never designed to have a torquey little (or big in some cases) motor bolted in there that wants to twist outself out whenever you twist the throttle.
The almost universal method of preventing the motor spinning out is by using an oversized axle with typically an M12 thread and then machining 2 flats into it with approximately 10mm spacing. This axle should then be fitted into a robust dropout with washers on both the inside and outside of the dropout. If you are using a fork designed for quick release hubs, this is where the first issue will arise. There will not be a flat surface to secure the larger than intended nut and washer on the outer face and an axle that is not correctly centred due to it's larger than standard radius. To address this you should use something like a c shaped washer that has been modofied to sit into the depression where the quick release hub would normally sit. Alternatively you could file the raised sections down so that there is sufficient sized flat section.
In addition to the above you should also fit a torque arm. This will take the very significant torque that is created and transfer that force to a point further away. By increasing the lever length, the force is reduced and you can quite securely mount the torque arm which will remove the twisting force applied to the dropouts. The torque arm should completely enclose the axle, so it has the added benefit that if the worst were to happen it would most likely keep the wheel in the forks, rather than bouncing down the road, while you kiss..... or fall in traffic.... 2 torque arms are better than 1 in this respect as if they are securely mounted to the fork, they'll keep the wheel in the fork no matter what happens to the dropouts. If you can't get hold of a torque arm you could use a 10mm spanner and secure it to the fork with a couple of jubilee clips as a temperory solution. Not the perfect solution, but better than nothing.
Finally you need to make sure the nuts are good and tight and make sure to check them periodically. Especially the first few days after fitting the wheel as the axle will move it a little until it's backed up to the torque arm.
There are alloy suspension forks manufactured that are intended for front hub motors. I'm gonna have some in the near future so will try to post some info and pics about them. I'm sure I'm being overly cautious and that with a little 250W motor, you should be OK. The same priciples should also be applied to rear motor installations.
Keep safe.
