It's better they ask usually, otherwise you end up with a house full of tat nobody wants (or naice wooden objects nobody wants), or duplicates.
Before my parents and in-laws started asking my mother would buy books which we already have or which were miles away from the children's taste/ too young for them and my mil would buy vast quantities of plastic toys - usually things the children did like, or on a theme they liked, but so much they were overwhelmed when younger.
Now the children are older it's easy to buy exactly the wrong version of something if you're not exact.
My kids are polite and express thanks but it's really crap if you only have a very small extended family and generous grandparents spend £50+ on something you really don't want, which takes up space and you can't give to the charity shop in case they want to see it...
I was given some absolute corkers of terrible presents as a child (along the lines of Ralphie's pink rabbit suit in 'A Christmas Story'
and expected to "perform" gratitude for extended periods and write long thank you letters extolling the virtues of the enormous bloomers, full length polyester nightdress with ditching, or lacy handkerchief monogrammed with someone else's initials... It's even worse when the same person sends your sibling something that they do want - like chocolate 
So I'd rather people ask, and I ask for the children I buy for too.