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 $203.64   12.741%   $229.5858   $229.59   $229.60   $229.60  
  Purchase #2 $226.89   11.356%   $252.6556   $252.66   $252.65   $252.70  
  Purchase #3 $147.77   12.846%   $166.7525   $166.75   $166.75   $166.80  
  Purchase #4 $220.65   12.279%   $247.7436   $247.74   $247.75   $247.70  
  Purchase #5 $29.86   9.708%   $32.7588   $32.76   $32.75   $32.80  
  Purchase #6 $159.39   5.647%   $168.3908   $168.39   $168.40   $168.40  
  Purchase #7 $88.47   7.492%   $95.0982   $95.10   $95.10   $95.10  
  7 purchase totals = $1,076.67       $1,192.9853   $1,192.99   $1,193.00   $1,193.10