Without wishing to proscribe how we should all collect, if you intend collecting back in time then doing so from the beginning alongside modern coins is not such a bad idea. It is easy to say I'll get the modern ones and finish them off before going backwards, but even in this area there are relatively expensive pieces to acquire. Go back a couple hundred years and identically graded pieces sell for an order or orders of magnitude more. We all suffer funding constraints, so if you are in for the long haul buy yourself a mixture of cheaper and more expensive pieces. If you get all the cheap things immediately (and there are a lot of them) you are likely to find that you have already spent a considerable sum making the acquisition of the next group a daunting task when you discover it will cost multiples of the first stage. Unless you intend restricting yourself to a finite and relatively narrow band, this problem will raise it's head earlier than you think. Very few people are fortunate or affluent enough in that order to even get close to completing an all encompassing denomination collection, short run oddballs such as quarter farthings excepted.