going up is not going to hurt your car (say if it recommended 91 octane,using 98 octane will still benefit your car,most of the time these petrols are the premium unleaded and are slightly more refined).

However using a lower octane rating than is recommended can cause damage to your engine.a manufacturer will usually recommend 95 or 98 (premium unleaded) on engine with higher compression ratios....the higher octane fuels burn more smoothly and evenly in these 'high performance engines' wheres 91 octane fuel could cause an engine condition known as pinging,this is very bad for your engine,will shorten its life and in extremes can melt holes in the top of your pistons....off topic,but another cause of pinging is timing being too far advanced. If your car pings,checking your timing,or going up an octane rating in fuel can fix it