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 $6.90   11.870%   $7.719   $7.72   $7.70   $7.70  
  Purchase #2 $8.75   13.661%   $9.9453   $9.95   $9.95   $9.90  
  Purchase #3 $43.91   13.455%   $49.8181   $49.82   $49.80   $49.80  
  Purchase #4 $116.43   8.238%   $126.0215   $126.02   $126.00   $126.00  
  Purchase #5 $122.33   10.811%   $135.5551   $135.56   $135.55   $135.60  
  Purchase #6 $189.89   6.673%   $202.5614   $202.56   $202.55   $202.60  
  Purchase #7 $66.66   13.506%   $75.6631   $75.66   $75.65   $75.70  
  7 purchase totals = $554.87       $607.2835   $607.29   $607.20   $607.30