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 $20.39   3.452%   $21.0939   $21.09   $21.10   $21.10  
  Purchase #2 $165.60   6.814%   $176.884   $176.88   $176.90   $176.90  
  Purchase #3 $20.66   5.843%   $21.8672   $21.87   $21.85   $21.90  
  Purchase #4 $98.37   5.217%   $103.502   $103.50   $103.50   $103.50  
  Purchase #5 $80.68   8.358%   $87.4232   $87.42   $87.40   $87.40  
  Purchase #6 $20.25   12.181%   $22.7167   $22.72   $22.70   $22.70  
  Purchase #7 $84.46   5.360%   $88.9871   $88.99   $89.00   $89.00  
  7 purchase totals = $490.41       $522.4741   $522.47   $522.45   $522.50