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 $24.52   9.382%   $26.8205   $26.82   $26.80   $26.80  
  Purchase #2 $128.93   7.311%   $138.3561   $138.36   $138.35   $138.40  
  Purchase #3 $23.81   10.674%   $26.3515   $26.35   $26.35   $26.40  
  Purchase #4 $176.47   13.235%   $199.8258   $199.83   $199.85   $199.80  
  Purchase #5 $203.28   3.203%   $209.7911   $209.79   $209.80   $209.80  
  Purchase #6 $184.35   9.859%   $202.5251   $202.53   $202.55   $202.50  
  Purchase #7 $235.41   9.545%   $257.8799   $257.88   $257.90   $257.90  
  7 purchase totals = $976.77       $1,061.5500   $1,061.56   $1,061.60   $1,061.60