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 $15.55   8.678%   $16.8994   $16.90   $16.90   $16.90  
  Purchase #2 $218.43   5.717%   $230.9176   $230.92   $230.90   $230.90  
  Purchase #3 $5.91   6.216%   $6.2774   $6.28   $6.30   $6.30  
  Purchase #4 $113.68   7.565%   $122.2799   $122.28   $122.30   $122.30  
  Purchase #5 $180.81   12.381%   $203.1961   $203.20   $203.20   $203.20  
  Purchase #6 $6.40   8.647%   $6.9534   $6.95   $6.95   $7.00  
  Purchase #7 $48.45   9.447%   $53.0271   $53.03   $53.05   $53.00  
  7 purchase totals = $589.23       $639.5509   $639.56   $639.60   $639.60