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 $99.88   5.640%   $105.5132   $105.51   $105.50   $105.50  
  Purchase #2 $106.64   9.727%   $117.0129   $117.01   $117.00   $117.00  
  Purchase #3 $160.68   8.837%   $174.8793   $174.88   $174.90   $174.90  
  Purchase #4 $129.98   10.577%   $143.728   $143.73   $143.75   $143.70  
  Purchase #5 $14.98   11.427%   $16.6918   $16.69   $16.70   $16.70  
  Purchase #6 $110.78   5.685%   $117.0778   $117.08   $117.10   $117.10  
  Purchase #7 $13.27   13.816%   $15.1034   $15.10   $15.10   $15.10  
  7 purchase totals = $636.21       $690.0064   $690.00   $690.05   $690.00