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 $240.92   3.839%   $250.1689   $250.17   $250.15   $250.20  
  Purchase #2 $117.57   11.151%   $130.6802   $130.68   $130.70   $130.70  
  Purchase #3 $61.34   7.567%   $65.9816   $65.98   $66.00   $66.00  
  Purchase #4 $76.45   5.226%   $80.4453   $80.45   $80.45   $80.40  
  Purchase #5 $167.85   4.847%   $175.9857   $175.99   $176.00   $176.00  
  Purchase #6 $121.40   12.209%   $136.2217   $136.22   $136.20   $136.20  
  Purchase #7 $148.85   8.189%   $161.0393   $161.04   $161.05   $161.00  
  7 purchase totals = $934.38       $1,000.5227   $1,000.53   $1,000.55   $1,000.50