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 $19.64   7.768%   $21.1656   $21.17   $21.15   $21.20  
  Purchase #2 $217.59   5.845%   $230.3081   $230.31   $230.30   $230.30  
  Purchase #3 $56.67   10.560%   $62.6544   $62.65   $62.65   $62.70  
  Purchase #4 $75.47   12.831%   $85.1536   $85.15   $85.15   $85.20  
  Purchase #5 $32.58   7.512%   $35.0274   $35.03   $35.05   $35.00  
  Purchase #6 $79.82   5.148%   $83.9291   $83.93   $83.95   $83.90  
  Purchase #7 $18.35   8.664%   $19.9398   $19.94   $19.95   $19.90  
  7 purchase totals = $500.12       $538.1780   $538.18   $538.20   $538.20