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 $170.83   11.199%   $189.9613   $189.96   $189.95   $190.00  
  Purchase #2 $203.45   8.383%   $220.5052   $220.51   $220.50   $220.50  
  Purchase #3 $27.40   12.818%   $30.9121   $30.91   $30.90   $30.90  
  Purchase #4 $9.46   5.428%   $9.9735   $9.97   $9.95   $10.00  
  Purchase #5 $128.67   11.670%   $143.6858   $143.69   $143.70   $143.70  
  Purchase #6 $73.53   13.226%   $83.2551   $83.26   $83.25   $83.30  
  Purchase #7 $87.37   9.323%   $95.5155   $95.52   $95.50   $95.50  
  7 purchase totals = $700.71       $773.8085   $773.82   $773.75   $773.90