It is usually "safe" to take a bonus when you intend ONLY to play slots with it. The many twists and turns of bonus conditions apply to non-slot games, and this is where the unwary player gets caught out. Quite a few casinos however ban play on PROGRESSIVE slot games, which CAN catch out the slots player.
As far as WR are concerned, slots have the least playthrough. Microgaming uses 30x bonus as a default, however this is operator adjustable, and I have seen this go as high as 80x bonus even on slots.
A recently introduced trap for the unwary are terms stipulating that no bet greater than a certain percentage of the bonus, balance, deposit, or combination thereof, may be made. This is usually pretty generous, and bets of 10% or less of the bonus granted should be safe. The trap is where a player has a big win, and starts raising the bet with the much higher balance - it is easy to forget the max bet limit, especially if you started off with a small bonus.
Free chips usually come with tighter restrictions than deposit related bonuses, often having a maximun cashout.
An interesting way to play slots WITHOUT a bonus might be to enter a reward based promotion, where you stand to win a prize for aspects of your play, such as highest wagering (the usual), most spins (32Red Slotsmeister), or even based on a lucky draw. This would enable you to cash out at any time if you win big, and have your reward bonus credited at the end of the promo. Whilst this will come with WR, the important thing is that this does NOT tie up your initial deposit.
Check the terms carefully though, some promotions that LOOK like reward contests are nothing more than fancy deposit bonuses. These will stipulate the bonus is credited at the end of the promotion, BUT will contain a term that states any cash-in before the end of the promotion voids the bonus.
The Jackpot Factory ongoing 10% offer is a prime example of this. If you cash-out before the 10% is credited, you don't get it for ANY deposits made in the qualifying period - even those you lost!
Kasino King is something of an expert in bonuses for low rollers, and has been very "lucky" over the past 5 years playing mostly slot bonus offers.