Join Date: May 2013
Location: 'stralia, whereh teh weahter canneh dehcide whet et wahnts te beh.
In general you try to keep as little air in the loop as possible, any air encourages corrosion and growth, but a little bit of bubbles in a reservoir is perfectly fine provided it stays there. Over time the bubbles will 'grow' due to the water evaporating, if this becomes an issue you can simply add an air trap (commonly the top-up line/hose) and top it up every now and then.
Generally this is why reservoirs are usually mounted vertically as it simplifies air management.