Thanks for your reply, Ruimar. Now I have a bit better idea of techniques you are familiar with!
Hinging (T/V/S) should be reversible. That way (as Prospero mentions above), if a customer ever needs to get an item reframed (change of taste, glass breaks and damages mount), the artwork/photo/whatever can be taken from the current frame. Personally, I would never ever apply anything to the front of the artwork, whether it's a cheap 'n' cheerful postcard or an original watercolour. As Prospero says, that is mutilation.
P90 is a good basic tape to use for everyday work, but conservation level work requires better. Get yourself a Lion catalogue: there are plenty of different types to choose from. Sellotape is for Christmas presents, and masking tape is for decorating! Lion also carry a range of educational books that will provide handy reference for you, especially while you are in the early stages.
The basics of cold dry mounting are straightforward, but great care is required to execute the technique successfully, particularly on larger pieces, and lightweight media. First assess whether the item you wish to dry mount is suitable for the process (value of piece, condition of piece). Assuming the piece is suitable, cut adhesive board to size and expose a small strip of adhesive surface, anchor artpiece to the strip, pull back release paper, smoothing down the artipiece as you go, then apply pressure (roller/press) to strengthen the bond. Successful cold dry mounting is tricky enough, but like all skills, gets more straightforward with practice.
I would recommend that you practice on some 'potentially disposable' or replaceable items of your own (small items first, then larger as your skill increases). I would respectfully advise you not to attempt this technique with customer work until you're confident in its use: the process is about as irreversible as it gets, and it's very unforgiving once it starts to go wrong. Lion have a book on laminating and mounting that might be worth adding to your reference library.