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 $227.82   13.825%   $259.3161   $259.32   $259.30   $259.30  
  Purchase #2 $94.44   10.681%   $104.5271   $104.53   $104.55   $104.50  
  Purchase #3 $108.95   11.335%   $121.2995   $121.30   $121.30   $121.30  
  Purchase #4 $135.31   13.559%   $153.6567   $153.66   $153.65   $153.70  
  Purchase #5 $119.31   10.225%   $131.5094   $131.51   $131.50   $131.50  
  Purchase #6 $233.77   12.131%   $262.1286   $262.13   $262.15   $262.10  
  Purchase #7 $204.96   5.495%   $216.2226   $216.22   $216.20   $216.20  
  7 purchase totals = $1,124.56       $1,248.6600   $1,248.67   $1,248.65   $1,248.60