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 $246.29   13.387%   $279.2608   $279.26   $279.25   $279.30  
  Purchase #2 $9.93   3.728%   $10.3002   $10.30   $10.30   $10.30  
  Purchase #3 $68.77   9.380%   $75.2206   $75.22   $75.20   $75.20  
  Purchase #4 $165.37   11.779%   $184.8489   $184.85   $184.85   $184.80  
  Purchase #5 $79.90   10.849%   $88.5684   $88.57   $88.55   $88.60  
  Purchase #6 $153.49   6.243%   $163.0724   $163.07   $163.05   $163.10  
  Purchase #7 $232.58   9.255%   $254.1053   $254.11   $254.10   $254.10  
  7 purchase totals = $956.33       $1,055.3766   $1,055.38   $1,055.30   $1,055.40