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 $18.35   11.302%   $20.4239   $20.42   $20.40   $20.40  
  Purchase #2 $8.96   6.701%   $9.5604   $9.56   $9.55   $9.60  
  Purchase #3 $91.98   8.160%   $99.4856   $99.49   $99.50   $99.50  
  Purchase #4 $219.65   3.329%   $226.9621   $226.96   $226.95   $227.00  
  Purchase #5 $28.96   9.386%   $31.6782   $31.68   $31.70   $31.70  
  Purchase #6 $128.91   7.577%   $138.6775   $138.68   $138.70   $138.70  
  Purchase #7 $135.72   3.283%   $140.1757   $140.18   $140.20   $140.20  
  7 purchase totals = $632.53       $666.9634   $666.97   $667.00   $667.10