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 $243.36   10.153%   $268.0683   $268.07   $268.05   $268.10  
  Purchase #2 $169.94   12.438%   $191.0771   $191.08   $191.10   $191.10  
  Purchase #3 $23.74   3.718%   $24.6227   $24.62   $24.60   $24.60  
  Purchase #4 $70.87   3.395%   $73.276   $73.28   $73.30   $73.30  
  Purchase #5 $165.77   8.244%   $179.4361   $179.44   $179.45   $179.40  
  Purchase #6 $112.28   8.673%   $122.018   $122.02   $122.00   $122.00  
  Purchase #7 $112.57   7.232%   $120.7111   $120.71   $120.70   $120.70  
  7 purchase totals = $898.53       $979.2093   $979.22   $979.20   $979.20