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 $12.31   4.708%   $12.8896   $12.89   $12.90   $12.90  
  Purchase #2 $98.44   7.822%   $106.14   $106.14   $106.15   $106.10  
  Purchase #3 $200.73   5.204%   $211.176   $211.18   $211.20   $211.20  
  Purchase #4 $160.64   11.422%   $178.9883   $178.99   $179.00   $179.00  
  Purchase #5 $229.78   9.536%   $251.6918   $251.69   $251.70   $251.70  
  Purchase #6 $19.59   7.120%   $20.9848   $20.98   $21.00   $21.00  
  Purchase #7 $53.40   6.134%   $56.6756   $56.68   $56.70   $56.70  
  7 purchase totals = $774.89       $838.5461   $838.55   $838.65   $838.60