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 $90.64   10.592%   $100.2406   $100.24   $100.25   $100.20  
  Purchase #2 $19.67   6.245%   $20.8984   $20.90   $20.90   $20.90  
  Purchase #3 $135.69   5.512%   $143.1692   $143.17   $143.15   $143.20  
  Purchase #4 $131.72   9.221%   $143.8659   $143.87   $143.85   $143.90  
  Purchase #5 $195.91   8.151%   $211.8786   $211.88   $211.90   $211.90  
  Purchase #6 $197.74   8.226%   $214.0061   $214.01   $214.00   $214.00  
  Purchase #7 $18.53   4.630%   $19.3879   $19.39   $19.40   $19.40  
  7 purchase totals = $789.90       $853.4467   $853.46   $853.45   $853.50