Apart from a position where it looks good, and you can see all the comings and goings, there are some other considerations in deciding where to put your birdbath. The RSPB have tips, and interestingly, placement it seems, will affect the type of bird visiting and using it. They also suggest a prickly solution to the problem of our feline friends.
Mine gets full of leaves and bits in no time, so needs a quick scrub and clean before refilling - but the birds love the tree above, congregating on the branches and chattering after their bathing.
I sought Gertrude Jekyll's advice on the subject, and found in the introduction to her book Garden Ornament, she writes 'It is hoped that these illustrations of all kinds of garden ornament, now brought together for convenient study and comparison, may serve, not only to quicken the interest in beautiful gardening but also to show how ornament may best be applied, according to the quality or calibre of any place. The descriptions and critical remarks are to be taken as suggestive rather than authoritative, but they will be useful in directing attention to the various objects and their judicious treatment, mainly as to the preservation of harmony and avoidance of incongruity'.
However, have found nothing specific relating to birdbaths - so I'll leave mine where it is!