"... Do all pre-1990 German pottery pieces designate E or W Germany? ..."
No. Even if it's a packet, this excerpt from the 'Historical Notes' section on my website at www.porcelainmarksa ndmore.com
could help you to understand why one can not generalize the markings in this context :Following the founding of the two German states in October 1949 the western manufacturers saw themselves confronted with a problem. At that time and under economic/quality aspects the reputation of everything marked 'Made in Germany' was very high, on the other hand it was known that the whole system of production in regions influenced by the Soviets was less than adequate. Believing that the split of the two states was no more than a temporary solution and one should hold open a back door, the manufacturers in the Federal Republic of Germany thought it would be a good idea to use 'Western Germany' as that was near enough to the old 'Germany' but also implied western world orientation and distanced them from the sub-standard Soviet-influenced East German side. The German Democratic Republic countered by starting to use 'Eastern Germany' and the 'name race' started with all its twists and (partially funny) turns. After the fall of the Wall in November 1989 and the official German reunification on October 3rd 1990 all companies nearly instantly used 'Made in Germany' again. The only thing you can be really sure about is that a product marked with any kind of East/West German designator can only have been made sometime between 1950 and 1990. No more, no less.
Against what is commonly claimed by various 'collectors' and 'experts' one should know that some East German companies at first continued to use '(Made in) Germany' even during their time in the German Democratic Republic; some even up until 1972. The interest of some sellers in keeping these facts low is obvious as many people want to sell their items off as been made before 1949 (and hence being more desireable) even if they were made much later. Now as for roughly pre-dating all these items, two guidelines apply:
 A first placement for an item of a single manufacturer can be found by taking into mind the general mark development over time, for example any given western manufacturer used a 'W.-Germany' mark later than a 'Made in West Germany' mark. Also remember that the prefix 'Made in' as part of the mark was at that time often seen optional so its use can change according to free space on the item, mark design or other preferences. All that can be boiled down to the following short and painless list, evolving over time from left to right:
- (Made in) Western Germany => (Made in) West Germany => (Made in) W.-Germany
- (Made in) Eastern Germany => (Made in) East Germany => (Made in) German Democratic Republic => (Made in) GDR
 Dating items is only possible when one knows the marking procedures of the specific company as each of them had their own internal timeline. For example some factories used 'Western Germany' on some items right up to 1976 while other manufacturers had long dropped that it in favour of 'West Germany' or 'W. Germany'.