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 $30.48   6.860%   $32.5709   $32.57   $32.55   $32.60  
  Purchase #2 $105.79   3.426%   $109.4144   $109.41   $109.40   $109.40  
  Purchase #3 $120.88   8.315%   $130.9312   $130.93   $130.95   $130.90  
  Purchase #4 $106.75   8.213%   $115.5174   $115.52   $115.50   $115.50  
  Purchase #5 $36.29   9.869%   $39.8715   $39.87   $39.85   $39.90  
  Purchase #6 $135.53   3.296%   $139.9971   $140.00   $140.00   $140.00  
  Purchase #7 $13.96   9.758%   $15.3222   $15.32   $15.30   $15.30  
  7 purchase totals = $549.68       $583.6247   $583.62   $583.55   $583.60