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 $19.50   3.333%   $20.1499   $20.15   $20.15   $20.10  
  Purchase #2 $30.52   11.585%   $34.0557   $34.06   $34.05   $34.10  
  Purchase #3 $174.39   4.165%   $181.6533   $181.65   $181.65   $181.70  
  Purchase #4 $49.85   4.440%   $52.0633   $52.06   $52.05   $52.10  
  Purchase #5 $64.35   5.767%   $68.0611   $68.06   $68.05   $68.10  
  Purchase #6 $141.63   8.276%   $153.3513   $153.35   $153.35   $153.40  
  Purchase #7 $165.99   5.202%   $174.6248   $174.62   $174.60   $174.60  
  7 purchase totals = $646.23       $683.9594   $683.95   $683.90   $684.10