hi Viv,
the general consensus is that if the cracks are small the chances are that they are just surface craks so probably no need to worry about them, Unless of course the bikes performance bogs down or runs crap then it may be a problem.
you could try spraying some WD 40 onto the rubbers with the bike ticking over and see if the engine tone changes or you can physically see the stuff being sucked into the cracks, if either of those happen then replace the rubbers.
I store my bike out of the elements 95 percent of the time as it seems to be sunshine that perishes the rubbers, as they crack from the outside inwards.