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 $121.32   6.262%   $128.9171   $128.92   $128.90   $128.90  
  Purchase #2 $109.46   9.703%   $120.0809   $120.08   $120.10   $120.10  
  Purchase #3 $109.59   9.700%   $120.2202   $120.22   $120.20   $120.20  
  Purchase #4 $7.98   3.849%   $8.2872   $8.29   $8.30   $8.30  
  Purchase #5 $39.34   5.853%   $41.6426   $41.64   $41.65   $41.60  
  Purchase #6 $32.88   5.588%   $34.7173   $34.72   $34.70   $34.70  
  Purchase #7 $249.91   10.277%   $275.5933   $275.59   $275.60   $275.60  
  7 purchase totals = $670.48       $729.4586   $729.46   $729.45   $729.40