Whatever your financial situation is or what your reception site tells you, it is NOT cool to have a cash bar! You would never ask your guests to pay for her chicken satay appetizer, you shouldn't ask guests to pay for drinks.
If money is tight or you are trying to cut down on costs just have beer and wine available or have a signature drink station. Maybe margaritas on the rocks to go along with the beer and wine or a maybe a Vodka bar. Think about your theme, time of year and location.
Along the same lines it is also very tacky to have tip jars or cups on the bar. Your guests should NOT have to tip for their drink at YOUR wedding. This note of etiquette also goes to my wedding guest readers - please don't tip the bartenders. A gratuity is included in the catering bill already. It is best to discuss this with your sales person prior to your wedding and of course your wedding planner will make sure there is not tip jar in view.