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 $236.95   8.407%   $256.8704   $256.87   $256.85   $256.90  
  Purchase #2 $25.78   7.349%   $27.6746   $27.67   $27.65   $27.70  
  Purchase #3 $103.61   11.599%   $115.6277   $115.63   $115.65   $115.60  
  Purchase #4 $53.79   5.438%   $56.7151   $56.72   $56.70   $56.70  
  Purchase #5 $219.25   3.219%   $226.3077   $226.31   $226.30   $226.30  
  Purchase #6 $61.42   3.196%   $63.383   $63.38   $63.40   $63.40  
  Purchase #7 $72.72   6.572%   $77.4992   $77.50   $77.50   $77.50  
  7 purchase totals = $773.52       $824.0777   $824.08   $824.05   $824.10