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 $33.92   4.564%   $35.4681   $35.47   $35.45   $35.50  
  Purchase #2 $37.65   5.687%   $39.7912   $39.79   $39.80   $39.80  
  Purchase #3 $12.57   9.410%   $13.7528   $13.75   $13.75   $13.80  
  Purchase #4 $116.94   3.445%   $120.9686   $120.97   $120.95   $121.00  
  Purchase #5 $91.77   7.658%   $98.7977   $98.80   $98.80   $98.80  
  Purchase #6 $18.90   8.807%   $20.5645   $20.56   $20.55   $20.60  
  Purchase #7 $230.68   10.855%   $255.7203   $255.72   $255.70   $255.70  
  7 purchase totals = $542.43       $585.0632   $585.06   $585.00   $585.20