It is better not to sign traveller's cheques BEFORE you use them. It is better to sign them in front of the person making the sale.
You would think that it is not in the interests of shopkeepers to steal from their customers. However, some shopkeepers or shop assistants have been known to operate a scam involving traveller's cheques when the cheques have been presented already signed.
The shop assistant asks to see your passport in order to compare signatures, then claims that the signatures do not seem to match. If the shop assistant then asks to take a photocopy of your passport and credit card to prove your identity, POLITELY REFUSE TO COMPLY.
Do not make any accusations. Preferably, leave the shop and make your purchase elsewhere. If that is not an option, pay by cash. Remember, the shopkeeper or assistant knows that you are a traveller and will be safely back in your own country before your credit card details are used inappropriately.
Of course, if you sign your cheques in front of the shop assistant, you should not have this problem.