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 $235.43   6.156%   $249.9231   $249.92   $249.90   $249.90  
  Purchase #2 $162.78   7.143%   $174.4074   $174.41   $174.40   $174.40  
  Purchase #3 $24.77   8.182%   $26.7967   $26.80   $26.80   $26.80  
  Purchase #4 $208.67   7.151%   $223.592   $223.59   $223.60   $223.60  
  Purchase #5 $91.52   3.137%   $94.391   $94.39   $94.40   $94.40  
  Purchase #6 $33.26   12.493%   $37.4152   $37.42   $37.40   $37.40  
  Purchase #7 $16.45   3.324%   $16.9968   $17.00   $17.00   $17.00  
  7 purchase totals = $772.88       $823.5222   $823.53   $823.50   $823.50