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 $52.45   12.299%   $58.9008   $58.90   $58.90   $58.90  
  Purchase #2 $154.40   7.271%   $165.6264   $165.63   $165.65   $165.60  
  Purchase #3 $245.97   3.769%   $255.2406   $255.24   $255.25   $255.20  
  Purchase #4 $207.85   13.370%   $235.6395   $235.64   $235.65   $235.60  
  Purchase #5 $21.80   7.463%   $23.4269   $23.43   $23.45   $23.40  
  Purchase #6 $123.28   12.453%   $138.6321   $138.63   $138.65   $138.60  
  Purchase #7 $220.72   8.654%   $239.8211   $239.82   $239.80   $239.80  
  7 purchase totals = $1,026.47       $1,117.2874   $1,117.29   $1,117.35   $1,117.10