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 $121.33   7.508%   $130.4395   $130.44   $130.45   $130.40  
  Purchase #2 $15.78   12.826%   $17.8039   $17.80   $17.80   $17.80  
  Purchase #3 $122.60   6.470%   $130.5322   $130.53   $130.55   $130.50  
  Purchase #4 $28.92   10.366%   $31.9178   $31.92   $31.90   $31.90  
  Purchase #5 $100.37   13.143%   $113.5616   $113.56   $113.55   $113.60  
  Purchase #6 $44.31   12.458%   $49.8301   $49.83   $49.85   $49.80  
  Purchase #7 $93.32   6.732%   $99.6023   $99.60   $99.60   $99.60  
  7 purchase totals = $526.63       $573.6874   $573.68   $573.70   $573.60