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 $106.67   12.444%   $119.944   $119.94   $119.95   $119.90  
  Purchase #2 $159.62   4.540%   $166.8667   $166.87   $166.85   $166.90  
  Purchase #3 $139.43   4.470%   $145.6625   $145.66   $145.65   $145.70  
  Purchase #4 $243.45   3.437%   $251.8174   $251.82   $251.80   $251.80  
  Purchase #5 $69.87   5.299%   $73.5724   $73.57   $73.55   $73.60  
  Purchase #6 $122.59   12.571%   $138.0008   $138.00   $138.00   $138.00  
  Purchase #7 $101.99   11.134%   $113.3456   $113.35   $113.35   $113.30  
  7 purchase totals = $943.62       $1,009.2094   $1,009.21   $1,009.15   $1,009.20