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 $248.51   3.418%   $257.0041   $257.00   $257.00   $257.00  
  Purchase #2 $156.35   11.255%   $173.9472   $173.95   $173.95   $173.90  
  Purchase #3 $172.39   11.665%   $192.4993   $192.50   $192.50   $192.50  
  Purchase #4 $241.46   10.584%   $267.0161   $267.02   $267.00   $267.00  
  Purchase #5 $249.39   9.825%   $273.8926   $273.89   $273.90   $273.90  
  Purchase #6 $28.92   10.566%   $31.9757   $31.98   $32.00   $32.00  
  Purchase #7 $184.59   10.536%   $204.0384   $204.04   $204.05   $204.00  
  7 purchase totals = $1,281.61       $1,400.3734   $1,400.38   $1,400.40   $1,400.30