ついでに

While having an opportunity..
Plain form Verb + ついでに
Noun + のついでに

ついでに means something like “While (I’m) at it…”.
ついでに attaches to the main goal and is followed by a less important action:
Main Goal AついでにMinor task B. 
It is important not to forget this even if ついでに is translated as “when”. 

Examples

• 買い物のついでに友達を会いに行った。
  When I was shopping, I met up with my friend. 
  
  
• 出勤するついでに、コンビニで弁当を買った。
  While going to work, I bought a lunch at the store. 
  
  
• 寺に行ったついでに、紅葉を見た
  I looked at the fall leaves when I went to the temple.*
  (Main reason for going to the temple was something else, like praying)

Even if not explicitly stated like in the 寺 example, ついでに implies a separate main goal. 

Tense

The tense for ついでに can be a little confusing. 
Here are some rules:

• The first verb may be either present or past tense:
  • 掃除したついでにゴミを出した 〇
    I took the trash out when I cleaned. 
    
    
  • 掃除するついでにゴミを出した 〇
    I took out the trash when I clean(ed). (Ok because of tense of last verb).
    
    
• unless it is completion past tense:
  
  • 家に帰ったついでに食べた 〇
    I ate when I had gone home.  
    
    
  • 家に帰るついでに食べた △
     I ate while going home. (Meaning is different) 
    
    
  • 近くに来たついでに挨拶に行った 〇
    When I was close by, I stopped by to say hi. 
    
    
  • 近くにくるついでに挨拶に行った ✖
    I stopped by to say hi when I go close by.  (“Coming close by” is not complete)
    
    
• The last verb will determine the overall tense
  • 家に帰ったついでに食べる
    I will eat when I have gone home (for some goal). 
    
    
  • 帰るついでに食べる
    I will eat while I am going home. (On the way home). 
    
    
  • 帰るついでに食べた
    I ate while I was going home. 
    
    

It may help to think of Present tense + ついでに as “While” and Past tense + ついでに as “When“

However in practice you probably won’t have to think about it too hard. 

References

Most helpful Japanese Yahoo Response
Weblio