That's a really good question. I'm interested in hearing what others have to say about the matter. My first thought is, yes, it's quite alright. But, I think proportion and practicality have to be considered. Extending the hearth beyond the width of the mantel by 5% is very different than extending it by 40%. If there are other furnishings flanking the fireplace a significant extension may not be practical. Here are a few of examples where the hearth and fireplace widths aren't the same, but the proportions are righ:

The choice of hearth depends on the style of your room decor as well as colour wise. Hearths can be bought ready to go from most fireplace suppliers in a range of colours and styles. Do you want to sit the fireplace on the hearth or on a ledge? We did not want the ready made angled style for our house. Our tiler did the lot from building & sheeting the hearth before tiling & sealing. We love it in our room setting of timber floor, timber ceiling and next to a timber wall.The consideration was fireproof materials and size/style of hearth guided by the building code. Other materials can be tile, glass, stone, concrete just have to be fire rated and professionally installed. Slate has been a popular choice forever and comes in different grades and colours but will need sealing most likely.

We were in the same dilemma recently. We opted a pre-made hearth to match the fire and a complete contrast to the floor. Hope you find a solution.

https://www.google.com.au/search?q=bt+bamboo+satin+finish&biw=1920&bih=888&source=lnms&tbm=isch&sa=X&ved=0ahUKEwis4pK2koLQAhUDp5QKHQXkCu0Q_AUIBigB#tbm=isch&q=bt+bamboo+satin+finish+cloud+colour&imgrc=Rtzqax6bGiapCM%3A