Most people ask me to use a ready made theme and fill it with their details (they choose a theme they like). Whereas others are okay with me using a theme and make changes to it. It's perfectly acceptable especially if your client isn't paying you the money to build the website from scratch which can take up to 3 months (a hefty price, I assure you). Although I wouldn't use a ready made theme without telling a client, I would explain the process to them beforehand ensuring they are 100% satisfied. Secondly I will show them the theme before and after my alterations so they can decide for themselves. End of. (I'M USING THIS METHOD AT THIS VERY MOMENT).