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 $10.92   11.620%   $12.1889   $12.19   $12.20   $12.20  
  Purchase #2 $198.62   4.608%   $207.7724   $207.77   $207.75   $207.80  
  Purchase #3 $103.92   10.516%   $114.8482   $114.85   $114.85   $114.80  
  Purchase #4 $15.45   8.507%   $16.7643   $16.76   $16.75   $16.80  
  Purchase #5 $25.52   5.207%   $26.8488   $26.85   $26.85   $26.80  
  Purchase #6 $210.98   6.293%   $224.257   $224.26   $224.25   $224.30  
  Purchase #7 $14.79   13.713%   $16.8182   $16.82   $16.80   $16.80  
  7 purchase totals = $580.20       $619.4978   $619.50   $619.45   $619.50