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 $130.47   6.385%   $138.8005   $138.80   $138.80   $138.80  
  Purchase #2 $49.40   7.542%   $53.1257   $53.13   $53.15   $53.10  
  Purchase #3 $42.71   13.430%   $48.446   $48.45   $48.45   $48.40  
  Purchase #4 $238.55   5.122%   $250.7685   $250.77   $250.75   $250.80  
  Purchase #5 $218.42   13.717%   $248.3807   $248.38   $248.40   $248.40  
  Purchase #6 $9.62   11.110%   $10.6888   $10.69   $10.70   $10.70  
  Purchase #7 $244.35   8.405%   $264.8876   $264.89   $264.90   $264.90  
  7 purchase totals = $933.52       $1,015.0978   $1,015.11   $1,015.15   $1,015.10