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 $233.75   6.700%   $249.4113   $249.41   $249.40   $249.40  
  Purchase #2 $82.83   10.260%   $91.3284   $91.33   $91.35   $91.30  
  Purchase #3 $223.46   12.200%   $250.7221   $250.72   $250.70   $250.70  
  Purchase #4 $140.94   4.298%   $146.9976   $147.00   $147.00   $147.00  
  Purchase #5 $178.52   13.255%   $202.1828   $202.18   $202.20   $202.20  
  Purchase #6 $108.63   4.430%   $113.4423   $113.44   $113.45   $113.40  
  Purchase #7 $219.42   7.474%   $235.8195   $235.82   $235.80   $235.80  
  7 purchase totals = $1,187.55       $1,289.9040   $1,289.90   $1,289.90   $1,289.80