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 $120.27   13.755%   $136.8131   $136.81   $136.80   $136.80  
  Purchase #2 $51.43   4.857%   $53.928   $53.93   $53.95   $53.90  
  Purchase #3 $23.26   5.108%   $24.4481   $24.45   $24.45   $24.40  
  Purchase #4 $170.32   5.126%   $179.0506   $179.05   $179.05   $179.10  
  Purchase #5 $243.75   6.459%   $259.4938   $259.49   $259.50   $259.50  
  Purchase #6 $89.71   9.213%   $97.975   $97.98   $98.00   $98.00  
  Purchase #7 $167.29   5.731%   $176.8774   $176.88   $176.90   $176.90  
  7 purchase totals = $866.03       $928.5860   $928.59   $928.65   $928.60