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 $249.40   12.683%   $281.0314   $281.03   $281.05   $281.00  
  Purchase #2 $129.96   8.783%   $141.3744   $141.37   $141.35   $141.40  
  Purchase #3 $119.85   6.262%   $127.355   $127.36   $127.35   $127.40  
  Purchase #4 $88.52   7.825%   $95.4467   $95.45   $95.45   $95.40  
  Purchase #5 $169.94   7.403%   $182.5207   $182.52   $182.50   $182.50  
  Purchase #6 $249.33   8.455%   $270.4109   $270.41   $270.40   $270.40  
  Purchase #7 $243.61   4.494%   $254.5578   $254.56   $254.55   $254.60  
  7 purchase totals = $1,250.61       $1,352.6969   $1,352.70   $1,352.65   $1,352.70