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 $29.79   5.336%   $31.3796   $31.38   $31.40   $31.40  
  Purchase #2 $39.34   6.672%   $41.9648   $41.96   $41.95   $42.00  
  Purchase #3 $164.79   9.229%   $179.9985   $180.00   $180.00   $180.00  
  Purchase #4 $183.87   11.707%   $205.3957   $205.40   $205.40   $205.40  
  Purchase #5 $10.58   13.503%   $12.0086   $12.01   $12.00   $12.00  
  Purchase #6 $181.60   11.244%   $202.0191   $202.02   $202.00   $202.00  
  Purchase #7 $51.35   8.197%   $55.5592   $55.56   $55.55   $55.60  
  7 purchase totals = $661.32       $728.3255   $728.33   $728.30   $728.40