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 $72.39   10.870%   $80.2588   $80.26   $80.25   $80.30  
  Purchase #2 $19.97   3.774%   $20.7237   $20.72   $20.70   $20.70  
  Purchase #3 $210.87   12.772%   $237.8023   $237.80   $237.80   $237.80  
  Purchase #4 $249.76   3.499%   $258.4991   $258.50   $258.50   $258.50  
  Purchase #5 $194.42   5.703%   $205.5078   $205.51   $205.50   $205.50  
  Purchase #6 $39.76   9.857%   $43.6791   $43.68   $43.70   $43.70  
  Purchase #7 $215.31   8.820%   $234.3003   $234.30   $234.30   $234.30  
  7 purchase totals = $1,002.48       $1,080.7711   $1,080.77   $1,080.75   $1,080.80