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 $189.89   6.757%   $202.7209   $202.72   $202.70   $202.70  
  Purchase #2 $232.64   10.747%   $257.6418   $257.64   $257.65   $257.60  
  Purchase #3 $58.29   4.375%   $60.8402   $60.84   $60.85   $60.80  
  Purchase #4 $199.33   7.782%   $214.8419   $214.84   $214.85   $214.80  
  Purchase #5 $96.95   11.317%   $107.9218   $107.92   $107.90   $107.90  
  Purchase #6 $123.97   5.814%   $131.1776   $131.18   $131.20   $131.20  
  Purchase #7 $9.81   4.251%   $10.227   $10.23   $10.25   $10.20  
  7 purchase totals = $910.88       $985.3712   $985.37   $985.40   $985.20