Nope no modification needed as long as the width size is not any larger then 8 or 8.5 if you plan on tucking them in.
Here's when you may need modification. I can say though at the most an 8.5 can fit and would be just enough to roll in the fenders with special offset, proper custom suspension upgrades, and perfect tire size, this would be pushing the limit though on a stock Celica. It's all about the offset as well such as a 35 all around making sure you get the perfect tire size as well. I've got 19x9's all around and slight modification rolling in the fenders and shaving down of the inner fender walls was needed to rid it from rubbing or any type of scraping to the tires.
Anyways most companies sell rims in their catalog factory tested to fit the Celica before selling & listing. The two most common widths usually listed in a diameter of 18 are 7 & 7.5 this would drop perfect and tuck in the fenders will not problem check the catalog first and if your desired width is not their then it's not reccomended for the car. Depends on the type of rims you plan on buying it's all different with different makers you've got the 1-2-3 piece rims you know what I mean