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 $234.87   12.821%   $264.9827   $264.98   $265.00   $265.00  
  Purchase #2 $68.28   5.183%   $71.819   $71.82   $71.80   $71.80  
  Purchase #3 $131.66   6.320%   $139.9809   $139.98   $140.00   $140.00  
  Purchase #4 $31.99   11.465%   $35.6577   $35.66   $35.65   $35.70  
  Purchase #5 $223.68   7.240%   $239.8744   $239.87   $239.85   $239.90  
  Purchase #6 $32.87   4.433%   $34.3271   $34.33   $34.35   $34.30  
  Purchase #7 $103.47   6.267%   $109.9545   $109.95   $109.95   $110.00  
  7 purchase totals = $826.82       $896.5963   $896.59   $896.60   $896.70