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 $37.75   6.411%   $40.1702   $40.17   $40.15   $40.20  
  Purchase #2 $136.60   6.694%   $145.744   $145.74   $145.75   $145.70  
  Purchase #3 $242.53   3.232%   $250.3686   $250.37   $250.35   $250.40  
  Purchase #4 $56.97   3.244%   $58.8181   $58.82   $58.80   $58.80  
  Purchase #5 $198.79   9.601%   $217.8758   $217.88   $217.90   $217.90  
  Purchase #6 $142.95   12.305%   $160.54   $160.54   $160.55   $160.50  
  Purchase #7 $141.72   8.578%   $153.8767   $153.88   $153.90   $153.90  
  7 purchase totals = $957.31       $1,027.3934   $1,027.40   $1,027.40   $1,027.40