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 $138.95   12.568%   $156.4132   $156.41   $156.40   $156.40  
  Purchase #2 $216.60   6.253%   $230.144   $230.14   $230.15   $230.10  
  Purchase #3 $103.99   5.625%   $109.8394   $109.84   $109.85   $109.80  
  Purchase #4 $74.28   6.575%   $79.1639   $79.16   $79.15   $79.20  
  Purchase #5 $222.41   12.428%   $250.0511   $250.05   $250.05   $250.10  
  Purchase #6 $29.52   12.447%   $33.1944   $33.19   $33.20   $33.20  
  Purchase #7 $10.39   11.856%   $11.6218   $11.62   $11.60   $11.60  
  7 purchase totals = $796.14       $870.4278   $870.41   $870.40   $870.40