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 $35.51   8.112%   $38.3906   $38.39   $38.40   $38.40  
  Purchase #2 $165.87   8.427%   $179.8479   $179.85   $179.85   $179.80  
  Purchase #3 $207.65   11.176%   $230.857   $230.86   $230.85   $230.90  
  Purchase #4 $230.73   6.266%   $245.1875   $245.19   $245.20   $245.20  
  Purchase #5 $96.57   8.368%   $104.651   $104.65   $104.65   $104.70  
  Purchase #6 $29.49   5.715%   $31.1754   $31.18   $31.20   $31.20  
  Purchase #7 $192.38   7.597%   $206.9951   $207.00   $207.00   $207.00  
  7 purchase totals = $958.20       $1,037.1045   $1,037.12   $1,037.15   $1,037.20