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 $74.40   7.517%   $79.9926   $79.99   $80.00   $80.00  
  Purchase #2 $124.54   11.354%   $138.6803   $138.68   $138.70   $138.70  
  Purchase #3 $15.25   11.635%   $17.0243   $17.02   $17.00   $17.00  
  Purchase #4 $74.34   12.581%   $83.6927   $83.69   $83.70   $83.70  
  Purchase #5 $134.92   3.597%   $139.7731   $139.77   $139.75   $139.80  
  Purchase #6 $143.78   3.831%   $149.2882   $149.29   $149.30   $149.30  
  Purchase #7 $174.66   8.740%   $189.9253   $189.93   $189.95   $189.90  
  7 purchase totals = $741.89       $798.3765   $798.37   $798.40   $798.40