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 $20.55   6.844%   $21.9564   $21.96   $21.95   $22.00  
  Purchase #2 $99.48   13.419%   $112.8292   $112.83   $112.85   $112.80  
  Purchase #3 $149.33   13.821%   $169.9689   $169.97   $169.95   $170.00  
  Purchase #4 $165.28   8.676%   $179.6197   $179.62   $179.60   $179.60  
  Purchase #5 $181.65   10.235%   $200.2419   $200.24   $200.25   $200.20  
  Purchase #6 $175.56   5.403%   $185.0455   $185.05   $185.05   $185.00  
  Purchase #7 $63.51   8.745%   $69.0639   $69.06   $69.05   $69.10  
  7 purchase totals = $855.36       $938.7255   $938.73   $938.70   $938.70