The time of year definitely matters.
Brides with spring wedding dates should book as far in advance as possible, as spring is also Prom season for limo companies, which means that inventory sells out fast. Limo prices can rise as much as 30% and have longer required minimum times. Up to a year in advance is recommended for a spring bride if you live in a major city.
Summer, especially June, is a busy time for weddings and limo inventory may book fast. The same goes for early fall (September and early October), which are becoming popular wedding months. Be sure to allow extra time before your wedding date to book transportation.
Winter weddings may require a more rugged vehicle (like an SUV either in stretch or classic) depending on the weather where you are. Be sure to start your search early so you're prepared.