The idea behind loosening the cam belt pulleys is so that you can apply an even, consistent tension ove the length of the belt. When I 1st did the job I couldn't work out how too release the cam pulleys off their taper lock so I did the same thing as you. I don't think it's absolutely critical but it does depend on the individual circumstances.
With regard to your problem I'd be check the connections to the coil packs and injectors, the condition and colour of the spark plugs and finally the cam timing. I would also refrain from reving it too hard just in case it is cam timing that's causing the problem.