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 $53.53   7.393%   $57.4875   $57.49   $57.50   $57.50  
  Purchase #2 $223.93   11.553%   $249.8006   $249.80   $249.80   $249.80  
  Purchase #3 $94.67   3.396%   $97.885   $97.89   $97.90   $97.90  
  Purchase #4 $200.41   3.318%   $207.0596   $207.06   $207.05   $207.10  
  Purchase #5 $220.60   13.562%   $250.5178   $250.52   $250.50   $250.50  
  Purchase #6 $19.64   4.355%   $20.4953   $20.50   $20.50   $20.50  
  Purchase #7 $186.93   9.161%   $204.0547   $204.05   $204.05   $204.10  
  7 purchase totals = $999.71       $1,087.3005   $1,087.31   $1,087.30   $1,087.40