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 $241.89   11.840%   $270.5298   $270.53   $270.55   $270.50  
  Purchase #2 $25.55   8.443%   $27.7072   $27.71   $27.70   $27.70  
  Purchase #3 $208.29   5.220%   $219.1627   $219.16   $219.15   $219.20  
  Purchase #4 $185.36   6.547%   $197.4955   $197.50   $197.50   $197.50  
  Purchase #5 $201.29   4.582%   $210.5131   $210.51   $210.50   $210.50  
  Purchase #6 $159.78   3.773%   $165.8085   $165.81   $165.80   $165.80  
  Purchase #7 $99.85   13.770%   $113.5993   $113.60   $113.60   $113.60  
  7 purchase totals = $1,122.01       $1,204.8161   $1,204.82   $1,204.80   $1,204.80