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 $245.42   6.239%   $260.7318   $260.73   $260.75   $260.70  
  Purchase #2 $25.79   4.171%   $26.8657   $26.87   $26.85   $26.90  
  Purchase #3 $84.88   8.144%   $91.7926   $91.79   $91.80   $91.80  
  Purchase #4 $61.63   5.177%   $64.8206   $64.82   $64.80   $64.80  
  Purchase #5 $140.44   5.223%   $147.7752   $147.78   $147.80   $147.80  
  Purchase #6 $113.28   9.563%   $124.113   $124.11   $124.10   $124.10  
  Purchase #7 $102.97   6.593%   $109.7588   $109.76   $109.75   $109.80  
  7 purchase totals = $774.41       $825.8577   $825.86   $825.85   $825.90