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 $229.50   13.142%   $259.6609   $259.66   $259.65   $259.70  
  Purchase #2 $193.75   9.447%   $212.0536   $212.05   $212.05   $212.10  
  Purchase #3 $88.56   4.173%   $92.2556   $92.26   $92.25   $92.30  
  Purchase #4 $68.56   10.751%   $75.9309   $75.93   $75.95   $75.90  
  Purchase #5 $210.25   6.483%   $223.8805   $223.88   $223.90   $223.90  
  Purchase #6 $224.28   8.600%   $243.5681   $243.57   $243.55   $243.60  
  Purchase #7 $138.71   7.396%   $148.969   $148.97   $148.95   $149.00  
  7 purchase totals = $1,153.61       $1,256.3186   $1,256.32   $1,256.30   $1,256.50