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 $77.51   3.787%   $80.4453   $80.45   $80.45   $80.40  
  Purchase #2 $107.34   12.168%   $120.4011   $120.40   $120.40   $120.40  
  Purchase #3 $174.66   13.468%   $198.1832   $198.18   $198.20   $198.20  
  Purchase #4 $32.91   10.689%   $36.4277   $36.43   $36.45   $36.40  
  Purchase #5 $50.88   6.357%   $54.1144   $54.11   $54.10   $54.10  
  Purchase #6 $6.74   9.453%   $7.3771   $7.38   $7.40   $7.40  
  Purchase #7 $11.81   3.476%   $12.2205   $12.22   $12.20   $12.20  
  7 purchase totals = $461.85       $509.1693   $509.17   $509.20   $509.10