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 $83.79   12.161%   $93.9797   $93.98   $94.00   $94.00  
  Purchase #2 $53.55   3.646%   $55.5024   $55.50   $55.50   $55.50  
  Purchase #3 $130.57   5.814%   $138.1613   $138.16   $138.15   $138.20  
  Purchase #4 $144.28   11.128%   $160.3355   $160.34   $160.35   $160.30  
  Purchase #5 $62.73   9.741%   $68.8405   $68.84   $68.85   $68.80  
  Purchase #6 $119.55   12.567%   $134.5738   $134.57   $134.55   $134.60  
  Purchase #7 $8.26   7.363%   $8.8682   $8.87   $8.85   $8.90  
  7 purchase totals = $602.73       $660.2614   $660.26   $660.25   $660.30