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 $120.31   6.820%   $128.5151   $128.52   $128.50   $128.50  
  Purchase #2 $215.64   3.144%   $222.4197   $222.42   $222.40   $222.40  
  Purchase #3 $40.81   9.495%   $44.6849   $44.68   $44.70   $44.70  
  Purchase #4 $5.26   3.596%   $5.4491   $5.45   $5.45   $5.40  
  Purchase #5 $70.77   3.471%   $73.2264   $73.23   $73.25   $73.20  
  Purchase #6 $29.77   4.588%   $31.1358   $31.14   $31.15   $31.10  
  Purchase #7 $13.98   8.529%   $15.1724   $15.17   $15.15   $15.20  
  7 purchase totals = $496.54       $520.6034   $520.61   $520.60   $520.50