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 $233.31   13.756%   $265.4041   $265.40   $265.40   $265.40  
  Purchase #2 $162.43   7.319%   $174.3183   $174.32   $174.30   $174.30  
  Purchase #3 $34.25   4.660%   $35.8461   $35.85   $35.85   $35.80  
  Purchase #4 $250.97   13.159%   $283.9951   $284.00   $284.00   $284.00  
  Purchase #5 $168.38   8.547%   $182.7714   $182.77   $182.75   $182.80  
  Purchase #6 $14.28   12.851%   $16.1151   $16.12   $16.10   $16.10  
  Purchase #7 $110.95   9.760%   $121.7787   $121.78   $121.80   $121.80  
  7 purchase totals = $974.57       $1,080.2288   $1,080.24   $1,080.20   $1,080.20