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 $249.40   8.221%   $269.9032   $269.90   $269.90   $269.90  
  Purchase #2 $88.56   9.145%   $96.6588   $96.66   $96.65   $96.70  
  Purchase #3 $91.84   7.501%   $98.7289   $98.73   $98.75   $98.70  
  Purchase #4 $42.34   11.435%   $47.1816   $47.18   $47.20   $47.20  
  Purchase #5 $106.72   11.621%   $119.1219   $119.12   $119.10   $119.10  
  Purchase #6 $33.29   6.761%   $35.5407   $35.54   $35.55   $35.50  
  Purchase #7 $159.42   3.281%   $164.6506   $164.65   $164.65   $164.70  
  7 purchase totals = $771.57       $831.7857   $831.78   $831.80   $831.80