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 $105.70   3.135%   $109.0137   $109.01   $109.00   $109.00  
  Purchase #2 $31.56   9.516%   $34.5632   $34.56   $34.55   $34.60  
  Purchase #3 $228.49   10.352%   $252.1433   $252.14   $252.15   $252.10  
  Purchase #4 $47.66   6.546%   $50.7798   $50.78   $50.80   $50.80  
  Purchase #5 $19.41   4.791%   $20.3399   $20.34   $20.35   $20.30  
  Purchase #6 $87.51   9.337%   $95.6808   $95.68   $95.70   $95.70  
  Purchase #7 $77.74   4.650%   $81.3549   $81.35   $81.35   $81.40  
  7 purchase totals = $598.07       $643.8756   $643.86   $643.90   $643.90