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 $81.37   3.202%   $83.9755   $83.98   $84.00   $84.00  
  Purchase #2 $174.26   11.864%   $194.9342   $194.93   $194.95   $194.90  
  Purchase #3 $177.94   9.823%   $195.419   $195.42   $195.40   $195.40  
  Purchase #4 $249.75   13.112%   $282.4972   $282.50   $282.50   $282.50  
  Purchase #5 $213.25   5.676%   $225.3541   $225.35   $225.35   $225.40  
  Purchase #6 $124.56   11.214%   $138.5282   $138.53   $138.55   $138.50  
  Purchase #7 $220.39   13.437%   $250.0038   $250.00   $250.00   $250.00  
  7 purchase totals = $1,241.52       $1,370.7120   $1,370.71   $1,370.75   $1,370.70