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 $156.74   12.792%   $176.7902   $176.79   $176.80   $176.80  
  Purchase #2 $165.58   5.334%   $174.412   $174.41   $174.40   $174.40  
  Purchase #3 $66.31   11.109%   $73.6764   $73.68   $73.70   $73.70  
  Purchase #4 $27.79   7.792%   $29.9554   $29.96   $29.95   $30.00  
  Purchase #5 $184.83   9.225%   $201.8806   $201.88   $201.90   $201.90  
  Purchase #6 $98.66   10.307%   $108.8289   $108.83   $108.85   $108.80  
  Purchase #7 $69.93   12.116%   $78.4027   $78.40   $78.40   $78.40  
  7 purchase totals = $769.84       $843.9462   $843.95   $844.00   $844.00