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 $116.63   6.654%   $124.3906   $124.39   $124.40   $124.40  
  Purchase #2 $226.26   8.846%   $246.275   $246.28   $246.30   $246.30  
  Purchase #3 $61.27   6.397%   $65.1894   $65.19   $65.20   $65.20  
  Purchase #4 $73.55   5.441%   $77.5519   $77.55   $77.55   $77.60  
  Purchase #5 $41.65   12.655%   $46.9208   $46.92   $46.90   $46.90  
  Purchase #6 $94.75   13.526%   $107.5659   $107.57   $107.55   $107.60  
  Purchase #7 $128.59   7.695%   $138.485   $138.49   $138.50   $138.50  
  7 purchase totals = $742.70       $806.3786   $806.39   $806.40   $806.50