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 $243.33   6.444%   $259.0102   $259.01   $259.00   $259.00  
  Purchase #2 $148.47   9.544%   $162.64   $162.64   $162.65   $162.60  
  Purchase #3 $224.48   12.529%   $252.6051   $252.61   $252.60   $252.60  
  Purchase #4 $25.58   7.289%   $27.4445   $27.44   $27.45   $27.40  
  Purchase #5 $47.63   9.785%   $52.2906   $52.29   $52.30   $52.30  
  Purchase #6 $111.67   4.342%   $116.5187   $116.52   $116.50   $116.50  
  Purchase #7 $58.67   8.767%   $63.8136   $63.81   $63.80   $63.80  
  7 purchase totals = $859.83       $934.3227   $934.32   $934.30   $934.20