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 $8.34   7.788%   $8.9895   $8.99   $9.00   $9.00  
  Purchase #2 $108.74   11.407%   $121.144   $121.14   $121.15   $121.10  
  Purchase #3 $133.77   3.260%   $138.1309   $138.13   $138.15   $138.10  
  Purchase #4 $218.35   12.754%   $246.1984   $246.20   $246.20   $246.20  
  Purchase #5 $64.65   8.186%   $69.9422   $69.94   $69.95   $69.90  
  Purchase #6 $112.84   12.253%   $126.6663   $126.67   $126.65   $126.70  
  Purchase #7 $44.57   13.486%   $50.5807   $50.58   $50.60   $50.60  
  7 purchase totals = $691.26       $761.6520   $761.65   $761.70   $761.60