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 $237.51   12.265%   $266.6406   $266.64   $266.65   $266.60  
  Purchase #2 $16.87   9.138%   $18.4116   $18.41   $18.40   $18.40  
  Purchase #3 $170.90   13.477%   $193.9322   $193.93   $193.95   $193.90  
  Purchase #4 $153.47   12.625%   $172.8456   $172.85   $172.85   $172.80  
  Purchase #5 $34.83   12.831%   $39.299   $39.30   $39.30   $39.30  
  Purchase #6 $219.35   12.626%   $247.0451   $247.05   $247.05   $247.00  
  Purchase #7 $161.92   9.829%   $177.8351   $177.84   $177.85   $177.80  
  7 purchase totals = $994.85       $1,116.0092   $1,116.02   $1,116.05   $1,115.80