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 $239.67   5.126%   $251.9555   $251.96   $251.95   $252.00  
  Purchase #2 $49.64   8.503%   $53.8609   $53.86   $53.85   $53.90  
  Purchase #3 $168.82   7.779%   $181.9525   $181.95   $181.95   $182.00  
  Purchase #4 $85.77   13.702%   $97.5222   $97.52   $97.50   $97.50  
  Purchase #5 $68.57   11.400%   $76.387   $76.39   $76.40   $76.40  
  Purchase #6 $82.25   11.627%   $91.8132   $91.81   $91.80   $91.80  
  Purchase #7 $21.40   13.231%   $24.2314   $24.23   $24.25   $24.20  
  7 purchase totals = $716.12       $777.7227   $777.72   $777.70   $777.80