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 $19.85   6.621%   $21.1643   $21.16   $21.15   $21.20  
  Purchase #2 $142.55   5.794%   $150.8093   $150.81   $150.80   $150.80  
  Purchase #3 $6.54   10.759%   $7.2436   $7.24   $7.25   $7.20  
  Purchase #4 $32.51   4.668%   $34.0276   $34.03   $34.05   $34.00  
  Purchase #5 $250.80   10.726%   $277.7008   $277.70   $277.70   $277.70  
  Purchase #6 $125.95   12.577%   $141.7907   $141.79   $141.80   $141.80  
  Purchase #7 $170.52   11.163%   $189.5551   $189.56   $189.55   $189.60  
  7 purchase totals = $748.72       $822.2914   $822.29   $822.30   $822.30