Are idioms affected by tone changing rules?
Yes it should be pronounced as the 2nd tone because it's put before character with the 4th tone. The sandhi rule applies with idioms too.

The cases 一 should be pronounced as the 1st tone, i.e. shouldn't change the tone, are listed as the follows:


  
  单独念时。如：一，一、二、三
  用在一词或一句的末尾。如：统一祖国，唯一条件，单一成分，表里如一，不论三七二十一
  表示年、月、日。如：1911年1月1日，一二•九运动，九•一八事变
  表示序数。如：第一村，第一名，第一等，第一位
  


(1. when used alone; 2. when used at the end of the word or sentence; 3. when used in date; 4. when used as ordinal number with the meaning "first".)


  “一”在上述语境中，不管它后面跟的字是什么声调，“一”的读音都保持其原调阴平，不发生变调。特别要引起注意的是第四种语境，因为许多序数在表述时采用了省略“第”的格式，这时“一”字也应读原调。


(Under the above circumstances, no matter what the following character's tone is, 一 should keep its original tone (i.e. the 1st tone) and doesn't change.)

试析“一”声调的读法