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 $108.66   11.670%   $121.3406   $121.34   $121.35   $121.30  
  Purchase #2 $165.28   8.121%   $178.7024   $178.70   $178.70   $178.70  
  Purchase #3 $45.26   12.576%   $50.9519   $50.95   $50.95   $51.00  
  Purchase #4 $131.73   7.657%   $141.8166   $141.82   $141.80   $141.80  
  Purchase #5 $212.69   4.791%   $222.88   $222.88   $222.90   $222.90  
  Purchase #6 $55.42   9.562%   $60.7193   $60.72   $60.70   $60.70  
  Purchase #7 $167.95   7.727%   $180.9275   $180.93   $180.95   $180.90  
  7 purchase totals = $886.99       $957.3383   $957.34   $957.35   $957.30