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 $25.79   7.657%   $27.7647   $27.76   $27.75   $27.80  
  Purchase #2 $22.58   5.477%   $23.8167   $23.82   $23.80   $23.80  
  Purchase #3 $115.55   10.767%   $127.9913   $127.99   $128.00   $128.00  
  Purchase #4 $22.72   5.607%   $23.9939   $23.99   $24.00   $24.00  
  Purchase #5 $171.82   10.678%   $190.1669   $190.17   $190.15   $190.20  
  Purchase #6 $140.90   12.363%   $158.3195   $158.32   $158.30   $158.30  
  Purchase #7 $106.88   4.126%   $111.2899   $111.29   $111.30   $111.30  
  7 purchase totals = $606.24       $663.3429   $663.34   $663.30   $663.40