Depends what machine you are playing I suppose BWB's secret 7 always rolled in a 2 on your last spin so if a 1 holds and you only have one spin left hold and the 2 will roll in for bonus.
I find when playing Barcrest's Megazone (Multistake 10p/20p/30p play with £25 Jackpot), if you have nothing on the trail, it'll offer a hold with two un-numbered fruits and one fruit with +1. I hold all three reels as sometimes it'll let you hold four times in a row, getting you to the bonus and possible feature entry. Mostly gives respins or boosts, but you can end up quids in if it gives you the Selector!