My two cents worth from someone who has worked in hospitality for many years. In Santo Domingo, as a business you have to charge ITBIS to everyone. DGII have been all over businesses that offered not to charge it for cash, and even the Chinese and even Colmados charge it now. The tax stands for Impuesto sobre Transferencia de Bienes Industrializados y Servicios, which means transfer tax for manufactured goods and services. And it works exactly the same as VAT in the UK, and I think the same as the Sales Tax in US states. You collect it and you deduct any ITBIS on purchased goods. Every month on the 20th you have to send a report to DGII including the long number on every single receipt for purchased goods, and you have to pay them what you owe by the 20th.
So onto the tips. The Propina Legal is confusing for businesses because the law says it must be 10% and split between all staff who provide service. Which is wooly and causes no end of discussions and arguments - should the cleaner be given a split? Does the manager get double? Staff can, and do, go to the Secretaria (the ministry of labor) to file a complaint if organisations are not paying the full 10%, and the inspectors then come (and usually want a huge back-hander themselves). And staff in hospitality talk of little else other than the 10%, there are always always accusations that the organisation is robbing them an not paying them their 10%. It's important that the 10% doesn't apply to take-out and isn't charged on the ITBIS, so if you start with RD$ 100, you have RD$ 10 service charge and RD$ 18 ITBIS. It's not 10% of the overall (RD$ 128 = 12.8) which is what confuses a lot of staff. Try explaining that to a guy who can't add up, trust me I've had to do it many many many many times.
Now should you tip on top? As someone who works in the industry, my answer is only if you have really great service. The staff are getting 10% and if you start giving big tips on top then you're just creating problems as they will all expect it, and ask for it, and get p!ssed when they don't get it. Restaurant prices are already crazy high in the DR. If you're going to take a $900 burger (i.e $15) and then add 18% ITBIS, and then 10% propina legal, and then give a 20% tip on top, you get 900 x 1.28 = RD$ 1,382. So the price has gone up by 50%, and we end up with a US$25 burger which is now way too expensive for what it is.
And in the long run this doesn't end well. Everyone is already struggling this year, the AirBnB's which are the bottom feeders in the tourism market are pretty much empty and we are now in the highest of high season. The issue is that this place used to be cheap, now it's becoming quite expensive, and the overall quality of pretty much everything - and especially customer service - just doesn't stack up compared to other countries that tourists can go to.
Apologies for the long post but I do know a bit about this.