It is a good idea to have them at TDC because in order to change the seals you will need to remove the valve keepers. Once you remove the keepers, the only thing holding the valve up will be the compressed air in the cylinder. If you lose pressure in the cylinder for any reason and don't have the keepers in place, the valve will fall into the cylinder
. But if the piston is at TDC, the valve won't be able to fall into the cylinder if you lose pressure in the cylinder. (at least that is how it worked on my small block chebbie when I replaced the valve stem seals). I think I bought an adaptor for my compression gauge and a valve spring compressor from Summit for less than $25 bucks when I did it and it worked like a champ.