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 $120.39   11.871%   $134.6815   $134.68   $134.70   $134.70  
  Purchase #2 $66.84   4.773%   $70.0303   $70.03   $70.05   $70.00  
  Purchase #3 $106.88   12.515%   $120.256   $120.26   $120.25   $120.30  
  Purchase #4 $249.99   4.122%   $260.2946   $260.29   $260.30   $260.30  
  Purchase #5 $69.26   5.333%   $72.9536   $72.95   $72.95   $73.00  
  Purchase #6 $73.29   9.558%   $80.2951   $80.30   $80.30   $80.30  
  Purchase #7 $85.31   10.609%   $94.3605   $94.36   $94.35   $94.40  
  7 purchase totals = $771.96       $832.8716   $832.87   $832.90   $833.00