Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $68.32   12.411%   $76.7992   $76.80   $76.80   $76.80  
  Purchase #2 $226.82   10.145%   $249.8309   $249.83   $249.85   $249.80  
  Purchase #3 $35.90   9.450%   $39.2926   $39.29   $39.30   $39.30  
  Purchase #4 $214.93   11.220%   $239.0451   $239.05   $239.05   $239.00  
  Purchase #5 $77.47   11.159%   $86.1149   $86.11   $86.10   $86.10  
  Purchase #6 $158.58   9.114%   $173.033   $173.03   $173.05   $173.00  
  Purchase #7 $12.78   6.466%   $13.6064   $13.61   $13.60   $13.60  
  7 purchase totals = $794.80       $877.7221   $877.72   $877.75   $877.60