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 $244.50   13.684%   $277.9574   $277.96   $277.95   $278.00  
  Purchase #2 $224.51   10.851%   $248.8716   $248.87   $248.85   $248.90  
  Purchase #3 $174.75   7.278%   $187.4683   $187.47   $187.45   $187.50  
  Purchase #4 $37.94   6.763%   $40.5059   $40.51   $40.50   $40.50  
  Purchase #5 $162.35   9.490%   $177.757   $177.76   $177.75   $177.80  
  Purchase #6 $108.89   8.801%   $118.4734   $118.47   $118.45   $118.50  
  Purchase #7 $98.31   8.706%   $106.8689   $106.87   $106.85   $106.90  
  7 purchase totals = $1,051.25       $1,157.9025   $1,157.91   $1,157.80   $1,158.10