Pool heat for May ?
It really is a personal choice. On the whole I'd say it isn't necessary but certain folks do enjoy their pool a little warmer than others.
My advice would be to wait and see what the weather is doing when you are here. Most MC's will turn the heater on if it is cold but be forewarned that most MC's have a minimum time period and it does cost extra.
The problem with waiting is that if pool heat is needed you will not really get the benefit on your first day as it does take around 24 hours for the pool to get warmer. This is because the pool heater has to recycle 11-19,000 gallons (depending on pool size) a few times to generate the heat.
Also, if you are using the pool heater, it is vital that there is a pool blanket to keep the heat in during the night time when temperatures can drop. It won't be so necessary in May but it is essential at the moment.