training roses over trellis arch

Good evening Ladyatlunch and welcome,if you let them do their own thing you will end up with all your blooms at the top and none at the base,so yes train them horizontally
The reason:It is important to fan out the main stems as horizontal as possible . When the canes are bent over horizontally sap flow to the ends is restricted - encouraging the side buds to break into growth. If these in turn are trained to the horizontal position the rose will gradually spread upwards with branches and, hopefully, flowers all the way.:WINK1: hope this is of some help to you.