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 $90.90   10.535%   $100.4763   $100.48   $100.50   $100.50  
  Purchase #2 $25.69   5.211%   $27.0287   $27.03   $27.05   $27.00  
  Purchase #3 $47.74   11.378%   $53.1719   $53.17   $53.15   $53.20  
  Purchase #4 $101.54   7.526%   $109.1819   $109.18   $109.20   $109.20  
  Purchase #5 $121.36   5.626%   $128.1877   $128.19   $128.20   $128.20  
  Purchase #6 $106.95   12.128%   $119.9209   $119.92   $119.90   $119.90  
  Purchase #7 $150.91   12.707%   $170.0861   $170.09   $170.10   $170.10  
  7 purchase totals = $645.09       $708.0535   $708.06   $708.10   $708.10