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 $225.35   10.462%   $248.9261   $248.93   $248.95   $248.90  
  Purchase #2 $192.61   6.258%   $204.6635   $204.66   $204.65   $204.70  
  Purchase #3 $88.43   5.259%   $93.0805   $93.08   $93.10   $93.10  
  Purchase #4 $134.64   13.663%   $153.0359   $153.04   $153.05   $153.00  
  Purchase #5 $113.48   13.800%   $129.1402   $129.14   $129.15   $129.10  
  Purchase #6 $153.28   4.223%   $159.753   $159.75   $159.75   $159.80  
  Purchase #7 $12.62   5.648%   $13.3328   $13.33   $13.35   $13.30  
  7 purchase totals = $920.41       $1,001.9320   $1,001.93   $1,002.00   $1,001.90