IMPORTANT NOTE - This information was written several years ago when I was a Tesco employee. Systems and policies may have changed now so please be aware of this.
I think it is important that some buyers be-aware about some of the cavettes with vouchers for Tesco. One such problem is that it is standard for a cashier to check that the voucher name/clubcard number matches that of the customer's clubcard (which needs to be present to process Tesco vouchers), a way round this is to use the keyfob or a non-named clubcard.
The second point is that non-store specific vouchers are often only accepted to the value of 10% of the shopping and identical coupons are limited to one of each kind. Because of abuse some stores no longer accept vouchers for products not purchased, it may be wise to find out first before invested in coupons.
As an example, you cannot pick up £20 of wine and expect to pay with 20 £1 Pampers vouchers unless you are lucky enough to find a inexperienced cashier. Although if you can source many different non-Tesco coupons you can use them even if you haven't brought the product (again only up to 10% of the value of the shopping and only one of each type). Tesco coupons are product specific although in some cases they fail to work as expected and a supervisor can override the system and allow you to have the points/value, but be aware of dates (both valid to and from), flavours, sizes and specific store locations printed on coupons.
Tesco coupons carry anti fraud protection and use one time only codes so be aware of using copies, likewise a cashier should not accept an obviously copied of defaced voucher. The self-service tills require an assistant override to accept certain types of coupons and in some cases will not take coupons at all.
A final point is that if a seller has scanned online coupons as a picture on his listing, the one-time codes may have already been used by a user visiting the listing and by time you get the voucher, it may not work. Online vouchers and vouchers without barcodes will not work in Tesco (excluding government/luncheon vouchers). Check out the internet as many 'printable' coupons and online codes can be sourced.
Tesco does accept Sainsburies vouchers, but only in the format £X off of a £XX shop. I don't think the date matters although you may find the cashier will want to check anyway. Bear in mind Tesco are allowing the use of these vouchers to get you to not shop at Sainsburies so they take a profit loss but they are typically for large amounts (£40+). You will also be limited in how many of these you can use (no more than one per shop).
Luncheon vouchers may only be used on food items not including alcohol and must be presented with the book that they came in, you can use any amount of luncheon vouchers but will not get change.
Milk tokens are only for the named party/postcode area and are government issued, they have limited longetivety and are worth the value of 7 one pint of standard milk, currently this is £2.31 (7x33). You must buy standard milk (not dried or 'luxury brands'), typically you don't need to buy 7 pints but will not have any change refunded to you. Infant tokens are different and have specific conditions on them and are not usualy accepted for the for the value of the dried milk, in this instance it would be wiser to use them at a chemist/clinic.
This guide should not be taken as absolute certainty, many stores have different polices and the training of staff regarding these matters is varied. You may want to ask a superviser even if it is after your shop is complete (although it is easier during the transaction), presenting your vouchers upfront can help by giving the cashier time to get a superviser.
Update - I have noticed some trade of gift cards, this seems like a stupid idea to me as you have no idea of how much money is currently on the card or if the card is even active, this is a case of buying more money for less money, it just doesn't happen! Buying a complete stamp book and spending it in December will give you £1 extra though (49 stamps gives you 50£ during December, 49£ otherwise). Old style Tesco gift vouchers have anti-fraud protection and will not be exchanged for money to a greater value than 99p.