It depends on several factors two of which are (i) the compund you intend to cycle; and (ii) how long you have had the breast tissue growth.
If you have had the lump for many months then there is very little chance of reversing it using drugs.
Tamoxifen is a SERM, it is mainly effective against estrogen related gynecomastia. If you intend to cycle nandrolone or trenbolone then tamoxifen is not ideal. Instead it would be better to use one of the dopamine agonists (cabergoline, bromocriptine, pramipexole) to reduce your prolactin levels, as gynecomastia can also be caused by excess prolactin.
In any event, if you have had the lumps for many months then the best you can do pharmacologically is to minimise their growth on cycle. In my humble opinion, doing a cycle if you have pre-existing gynecomastia is foolish. You are risking giving yourself advanced gynecomastia. If you have sizeable lumps then you will be especially senstive to estrogen and prolactin (simply by virtue of the greater presence of tissue that expresses receptors for estrogen and prolactin) In the worst case scenario, any aesthetic improvement you achieve from the cycle will be negated by your ugly breast mounds.
Get your gynecomastia treated -- surgically if necessary -- and then do a cycle.