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 $37.75   12.748%   $42.5624   $42.56   $42.55   $42.60  
  Purchase #2 $94.98   12.462%   $106.8164   $106.82   $106.80   $106.80  
  Purchase #3 $133.40   8.743%   $145.0632   $145.06   $145.05   $145.10  
  Purchase #4 $107.84   7.595%   $116.0304   $116.03   $116.05   $116.00  
  Purchase #5 $80.68   3.488%   $83.4941   $83.49   $83.50   $83.50  
  Purchase #6 $179.77   6.468%   $191.3975   $191.40   $191.40   $191.40  
  Purchase #7 $173.70   11.155%   $193.0762   $193.08   $193.10   $193.10  
  7 purchase totals = $808.12       $878.4402   $878.44   $878.45   $878.50