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 $17.77   4.459%   $18.5624   $18.56   $18.55   $18.60  
  Purchase #2 $94.72   6.433%   $100.8133   $100.81   $100.80   $100.80  
  Purchase #3 $211.52   11.216%   $235.2441   $235.24   $235.25   $235.20  
  Purchase #4 $129.31   4.472%   $135.0927   $135.09   $135.10   $135.10  
  Purchase #5 $149.70   9.572%   $164.0293   $164.03   $164.05   $164.00  
  Purchase #6 $8.81   10.238%   $9.712   $9.71   $9.70   $9.70  
  Purchase #7 $18.97   7.846%   $20.4584   $20.46   $20.45   $20.50  
  7 purchase totals = $630.80       $683.9122   $683.90   $683.90   $683.90