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 $223.35   3.550%   $231.2789   $231.28   $231.30   $231.30  
  Purchase #2 $234.45   11.264%   $260.8584   $260.86   $260.85   $260.90  
  Purchase #3 $217.83   5.692%   $230.2289   $230.23   $230.25   $230.20  
  Purchase #4 $104.38   5.299%   $109.9111   $109.91   $109.90   $109.90  
  Purchase #5 $183.33   3.395%   $189.5541   $189.55   $189.55   $189.60  
  Purchase #6 $88.46   9.748%   $97.0831   $97.08   $97.10   $97.10  
  Purchase #7 $18.58   7.855%   $20.0395   $20.04   $20.05   $20.00  
  7 purchase totals = $1,070.38       $1,138.9540   $1,138.95   $1,139.00   $1,139.00