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 $7.70   3.530%   $7.9718   $7.97   $7.95   $8.00  
  Purchase #2 $14.25   13.810%   $16.2179   $16.22   $16.20   $16.20  
  Purchase #3 $148.63   9.195%   $162.2965   $162.30   $162.30   $162.30  
  Purchase #4 $136.37   12.732%   $153.7326   $153.73   $153.75   $153.70  
  Purchase #5 $110.58   13.773%   $125.8102   $125.81   $125.80   $125.80  
  Purchase #6 $49.81   11.244%   $55.4106   $55.41   $55.40   $55.40  
  Purchase #7 $91.58   8.280%   $99.1628   $99.16   $99.15   $99.20  
  7 purchase totals = $558.92       $620.6024   $620.60   $620.55   $620.60