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 $27.75   3.577%   $28.7426   $28.74   $28.75   $28.70  
  Purchase #2 $122.79   5.130%   $129.0891   $129.09   $129.10   $129.10  
  Purchase #3 $29.42   7.171%   $31.5297   $31.53   $31.55   $31.50  
  Purchase #4 $19.65   7.439%   $21.1118   $21.11   $21.10   $21.10  
  Purchase #5 $214.67   3.571%   $222.3359   $222.34   $222.35   $222.30  
  Purchase #6 $7.70   6.483%   $8.1992   $8.20   $8.20   $8.20  
  Purchase #7 $87.41   7.142%   $93.6528   $93.65   $93.65   $93.70  
  7 purchase totals = $509.39       $534.6611   $534.66   $534.70   $534.60