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 $19.39   13.439%   $21.9958   $22.00   $22.00   $22.00  
  Purchase #2 $30.40   7.440%   $32.6618   $32.66   $32.65   $32.70  
  Purchase #3 $61.73   7.624%   $66.4363   $66.44   $66.45   $66.40  
  Purchase #4 $110.57   6.467%   $117.7206   $117.72   $117.70   $117.70  
  Purchase #5 $145.74   9.389%   $159.4235   $159.42   $159.40   $159.40  
  Purchase #6 $34.60   12.850%   $39.0461   $39.05   $39.05   $39.00  
  Purchase #7 $177.37   13.632%   $201.5491   $201.55   $201.55   $201.50  
  7 purchase totals = $579.80       $638.8332   $638.84   $638.80   $638.70