Keep at least one extra laser toner cartridge on hand, and have the required replacement inkjet cartridges available.  There is nothing worse than to discover you've run out of either in the middle of a customers printing job... and the delays involved in having to replace a toner or inkjet cartridge in mid-task are counter-productive to good time management.  Be prepared to service, clean, and refill your printers at all times.

A selection of papers in letter and legal size should be in stock, available in white and ivory, plus other commonly used colors that you are compfortable about stocking.  White and ivory cardstock in 40 and 60lbs weights should also be in stock for running business cards.  It is more cost effective to use a cutting board and cardstock to create business cards than it is to rely on pre-perforated business card stock.  Cutting business cards from card stock also leaves a cleaner edge on the cards... pre-cut cards that depend on folding and tearing to separate usually have feathery edges and can be accidentally bent during the separation process.

Custom papers and stationery is not required stock, a sample of available designs (including colored catalogs) is all that you'll require, but you will need to secure a source for any stationery products you present to your clients.  If you work in laser foils or laminations, it is a good idea to keep a minimal stock of these items.  The colors used in laser foils is difficult to duplicate in catalogs, being able to see the exact color and texture of the foil is important for you client.  If you use thermal sensitive papers a sample of each color selection should be available to show the color change of each and exactly how sensitive the paper is, this product also available in cardstock weight and at least one example to demonstrate the weight is required.

If you create CD covers and labels, or do the J-card liners for cassettes, it is a good idea to have both examples of the completed work and the optional papers used.  You might want to keep a single package of photo quality paper and a premium white paper to work with.  If you do t-shirt designs you should have a stock of transfer papers available for immediate use.