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 $27.87   3.586%   $28.8694   $28.87   $28.85   $28.90  
  Purchase #2 $71.35   12.617%   $80.3522   $80.35   $80.35   $80.40  
  Purchase #3 $180.78   3.396%   $186.9193   $186.92   $186.90   $186.90  
  Purchase #4 $173.75   3.613%   $180.0276   $180.03   $180.05   $180.00  
  Purchase #5 $21.44   3.836%   $22.2624   $22.26   $22.25   $22.30  
  Purchase #6 $39.55   5.524%   $41.7347   $41.73   $41.75   $41.70  
  Purchase #7 $9.59   13.384%   $10.8735   $10.87   $10.85   $10.90  
  7 purchase totals = $524.33       $551.0391   $551.03   $551.00   $551.10