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 $109.72   9.731%   $120.3969   $120.40   $120.40   $120.40  
  Purchase #2 $195.35   7.579%   $210.1556   $210.16   $210.15   $210.20  
  Purchase #3 $211.33   3.427%   $218.5723   $218.57   $218.55   $218.60  
  Purchase #4 $208.46   9.518%   $228.3012   $228.30   $228.30   $228.30  
  Purchase #5 $127.97   7.434%   $137.4833   $137.48   $137.50   $137.50  
  Purchase #6 $140.66   7.228%   $150.8269   $150.83   $150.85   $150.80  
  Purchase #7 $31.67   7.257%   $33.9683   $33.97   $33.95   $34.00  
  7 purchase totals = $1,025.16       $1,099.7045   $1,099.71   $1,099.70   $1,099.80