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 $212.67   13.316%   $240.9891   $240.99   $241.00   $241.00  
  Purchase #2 $190.61   6.468%   $202.9387   $202.94   $202.95   $202.90  
  Purchase #3 $154.68   7.270%   $165.9252   $165.93   $165.95   $165.90  
  Purchase #4 $221.39   5.581%   $233.7458   $233.75   $233.75   $233.70  
  Purchase #5 $120.54   10.428%   $133.1099   $133.11   $133.10   $133.10  
  Purchase #6 $120.77   3.782%   $125.3375   $125.34   $125.35   $125.30  
  Purchase #7 $120.51   13.273%   $136.5053   $136.51   $136.50   $136.50  
  7 purchase totals = $1,141.17       $1,238.5515   $1,238.57   $1,238.60   $1,238.40