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 $241.71   6.654%   $257.7934   $257.79   $257.80   $257.80  
  Purchase #2 $149.84   13.311%   $169.7852   $169.79   $169.80   $169.80  
  Purchase #3 $68.81   13.159%   $77.8647   $77.86   $77.85   $77.90  
  Purchase #4 $169.59   11.311%   $188.7723   $188.77   $188.75   $188.80  
  Purchase #5 $163.75   8.119%   $177.0449   $177.04   $177.05   $177.00  
  Purchase #6 $5.52   12.849%   $6.2293   $6.23   $6.25   $6.20  
  Purchase #7 $174.80   7.437%   $187.7999   $187.80   $187.80   $187.80  
  7 purchase totals = $974.02       $1,065.2897   $1,065.28   $1,065.30   $1,065.30