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 $113.96   6.652%   $121.5406   $121.54   $121.55   $121.50  
  Purchase #2 $186.74   4.747%   $195.6045   $195.60   $195.60   $195.60  
  Purchase #3 $48.32   13.774%   $54.9756   $54.98   $55.00   $55.00  
  Purchase #4 $163.62   11.141%   $181.8489   $181.85   $181.85   $181.80  
  Purchase #5 $49.26   3.466%   $50.9674   $50.97   $50.95   $51.00  
  Purchase #6 $189.50   6.548%   $201.9085   $201.91   $201.90   $201.90  
  Purchase #7 $165.70   6.312%   $176.159   $176.16   $176.15   $176.20  
  7 purchase totals = $917.10       $983.0045   $983.01   $983.00   $983.00