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 $104.89   10.199%   $115.5877   $115.59   $115.60   $115.60  
  Purchase #2 $87.54   11.867%   $97.9284   $97.93   $97.95   $97.90  
  Purchase #3 $117.39   6.714%   $125.2716   $125.27   $125.25   $125.30  
  Purchase #4 $23.75   13.662%   $26.9947   $26.99   $27.00   $27.00  
  Purchase #5 $112.40   9.436%   $123.0061   $123.01   $123.00   $123.00  
  Purchase #6 $239.79   7.425%   $257.5944   $257.59   $257.60   $257.60  
  Purchase #7 $83.95   4.327%   $87.5825   $87.58   $87.60   $87.60  
  7 purchase totals = $769.71       $833.9654   $833.96   $834.00   $834.00