it depended on whether or not you had interesting backgrounds... if it was something graphically and technically 'simple' like say
manic miner
-or-
jet set willy 
...then no, as it was set against a prodominantly balck b/g and it used pixel collision, but with something like, say...
Then you'd definatley use a pixel mask...
Towards the tail end of game development on the zx we tended to use masks as par for the course regardless...
Incidentally, for those of you that don't know what a pixel mask is its where you'd have a repeated version of the sprite as a silhoetted version kept in memory, sometimes (usually) 1 pixel bigger than your sprite... this version of the sprite is printed to screen first to (effectively) CUT OUT a section of the screen where the actual 'detailed' version of the sprites is then placed. The use of which is obvious when you look at the screens I've shown above... in case it's not obivous, it so that in games like knight lore, head over heels etc, any details in the background don't merge with the sprite creating an ugly mess of merged pixels...
it's not so much of a problem these days as there are more than 'just' two colors... or 1-bit depth... ON or OFF... I look at things like that assassins creed these days and cry at just how far digital visualisation have come...
