Light socket adapters - are they safe?
It's all about the watts, dude!

The safety concern surrounding the use of adapters, power boards, and the like is as you mentioned: wattage.

Power strip (board) safety 101:


Add up the wattage of all loads (in your case, CFLs) plugged into the power strip.
Look at the power strip for a wattage rating, and take four-fifths (or 80%) of it as a derating factor to prevent the safety switch (circuit breaker) from nuisance tripping.
Compare that four-fifths-of-the-rating number with the total wattage of your loads; if it's less than that, you're good to go!  If it's greater than that, you need to use more than one power strip/tap/board plugged into different outlets, or even multiple circuits coming from the panel.


Using a 1200W (240V 5A or 120V 10A) power strip/board and your example load of 5 30W CFLs:


The total load is 5*30W = 150W
The power board is rated for 1200W -- but we don't want to push it that far; the 80% is a derating factor as fuses and circuit breakers aren't exactly fans of operating at their rating continuously.  So, 80% of 1200W is 960W.
Since 960W (the amount of power the board should be handling on a continuous basis) is well over 150W, you are fine.